The Stovebolt Site Forums The Shop Area Engine Shop Gas milage on the 235's

Hi,
I purchased a 1962 Chevy C10, 235 strait 6, 4 speed in the floor witha granny gear and a single barrel carb.. Been running it about 4 months now and I love it.. I've already done a bunch of stuff to it to keep it a daily drive and road worthy.. New head, new carb, converted generator to alternator, electronic iginition coversion..
I'm into keeping it stock as much as possible... My question is: Does anyone know what kind of gas milage these trucks are suppose to be getting? I've talked with a handful of people that have owned them with these engines in them and they've all said they got between 15 and 20 miles per gallon.. I have been getting 11 miles per gallon.. What am I doing wrong and what can I do to adjust or improve this?
I took it to a rear end guy and he said I'm running a 4:10 ring and pinion in the back which is pretty low.. He suggested perhaps running a higher ring and pinion like a 3:72 or a 3:55 which would bring my rpms down a bit.. But i'm not so sure that is why i am getting poor milage.. Any suggestions or imput?
Thanks, Richie

The guy who told you to use a different rearend gear is right.When it comes to fuel mileage,gearing is the most important thing involved.So you either go to a higher gear ratio or an overdrive trans,to keep the engine RPMS down at cruising speed.If you are just using it for transportation a 3:08 rear or an overdrive transmission should do the trick.I am running a 350 Chev SB with a 700R4 trans.and 3:90 gears and getting 20 MPG.The 700R4 being an overdrive trans.which is turning the rearend at about .78 to 1 ratio.

i have a 54/3100/235 that i just recently overhauled, 435 miles on it. i am getting 12 mpg with fenton headers, dual rochesters and a t-5 with a 2.73 ring and pinion out of a nova. also i have a mini hei distributor. the lower gear will help. do a search and you will get a lot of different opinions of what will help. i having my carbs overhauled at this moment, one was new and the other is one that i overhualed years ago. i am hoping mine will get up into the 15 to 20 as the engine breaks in. i check years ago the mileage before i did all the work i have done and i got 12.5 i was hoping the upgrades would improve it. good luck. ron btw go the post in engine and drive line about holley weber disappointment.

Hey guys, thanks a bunch.. Definitely I am into changing out the ring and pinion.. I do about 50 percent freeway driving and i don't go much past 60 miles per hour and my tack is at about 3000 rpms at that speed..
My rear end guy says that I can't go any higher than a 3.72 ring and pinion unless I change out my carrier gizmo (cant remember the exact term he used..) Also he said that I would be very dissatisfied with the vehicle if I raised it up any higher than that.. He seems to think that there is a big difference between the 4.10 ratio and the 3.72 and that I will more than likely be happy with it..
I basically use the truck daily with only very light transporting.. Light ladders and some paint cans.. So I'm not hauling heavy stuff daily nor do I plan on it..
My main objective is to get it up to 15-20 miles per gallon without having to completely re-mod.. If the ring and pinion is the answer do you feel the 3.72 will fill the bill or do I need to take other steps?

If it were me:
Personally I like a tight rear, 4:11 or 3:90's.
First, I would want to know what is the ideal power curve for this motor, ideal, max hp, and torque, RPM's, etc. Chevy can give you that. I would work everything toward it.
Second, What size tires am I running? How about a taller tire and 16 inch rims for the six lugs are everywhere. I personally would shoot for 31" tall.
Third, I would go for a different tranny. One with an OD. Can take advantage of the 4:11 on take off and cruising on highway OD would run less than 1:1, maybe .72'ish. This should put my 70 MPH RPM at 2200 or there abouts.
If I have to downshift to pass a car I would consider more and progressive carboration.
Cavebull

Crap..! I'm confused.. The bigger tire idea was first suggested to me by my rear end guy.. I have the 15 inch rims on there now. I'm sure they came with the truck.. Actually, this truck was pretty un-bastardized when I received it.. I wanted to get some bigger rims that looked more stock but I couldn't find any.. Partially because I really didn't know what I was looking for or what to call them.. This is all new to me..
I looked in various catalogs that have Chevy parts and they all only gave 4 options for the ring and pinion replacements on my particular model engine and transmission.. The 3:90 wasn't one of them.. 4:10, 3:72, 3:55 and a really low one around 4:50ish..
Also, If I upped the rim and tire size to 16 inch rims and a 31 inch tire, I would have to re-calibrate my speed and odometer.. Which I'm fine with..
So I guess the three questions I have for this avenue would be: Would sizing up the rim and tires and re-calibrating the speed/odometer save me gas mileage? And would it be cheaper than having the ring and pinion changed out? And finally, could I still get those old school half moon hub caps on a 16 inch rim? I assume Chevy sold 16 inch rims as an option in those days?
The transmission change and or the overdrive options seem like they would be a big expense and a bit more than I would want to put into this vehicle at this point..
And finally, as far as obtaining the specs from Chevy, if you had a link I could hit up for that I would love it..
Thanks again.. I'm not a real mechanic and I really only know a little.. So I appreciate help and input from this forum.. Hell..!! My rear end guy told me to go on here and he's suppose to be an expert..

Going to tall 16" tires and rims should help mileage some. I don't think 16" was offered on 1/2 ton during that time period but you might check a wrecking yard and look for 16" 6 lug steelies off a late model GM utility truck. I think the 6 lug bolt patterns are compatible but not positive. If you could run 31" inch tires (215X85X16 for example) that would drop your 60mph RPM by about 300 RPM for not much $$. Short of that, I'd be checking the local vintage bone yards for that ring gear carrier you need to run 3.55 gears.

-Ed

You might be thinking about this too much. Cavebull is exactly right. To reword his comment, just find out at what RPM your engine delivers the highest fuel efficiency, draw out your driveline from engine to tire and pick the setup that puts your engine at that RPM when you're going 70mph or whatever your average cruising speed is.

You can use this formula (all unit conversions included) to figure out what your engine RPM will be:

(mph) x 336.1 x (diff. ratio) x (trans. ratio) / D

where D is your tire diameter in inches
diff. ratio is your 4.1 ring to pinion ratio in your rear diff.
and trans. ratio is your 4th gear ratio (1:1 for SM420, so you can leave it out of the equation)

I haven't looked it up but I'd shoot for around 1800-2200 RPM for best fuel economy.

Then there's fine tuning the engine...

(Carb. adjustment, timing adjustment, rebuild the engine, etc.)

Also, I rebuilt my SM420 trans. and just turning it by hand you can really feel the huge frictional losses through the transmission. I've also got a later 350 automatic trans. sitting around that had much less friction. Everyone's is different, though. You can also check your brakes to make sure they're not rubbing too much while disengaged.

Even simpler, you can get a 31 to 32 inch tall stock looking tire for the 15" rim too. I know for a fact a ~32" in bias tire from Firestone, and they are, IMHO, "stock cool!". Dont forget, radials help gas milage too, they say. I came within signing the check on that purchase. They are narrow, 6.5" wide. I like the look myself and the hub cap will work.
Per 6 lugs on 15 or 16's are no problem... all same. I have 54 6 lug truck rims on my 46.

One more easy thing to consider... the pumpkin in your rear swaps the same axels from 46 to 62 or 63. You can pull an old pumkin from a junk yard for next to nothing and have gears put in it. Eliminates down time on the truck and you get to keep the tight gear set for saturday nights. Poor mans quick change. I had a 67 and went to an earlier rear for this reason. Had a pumkin with 4:11's and a set of wide 50's for the weekends. Sunday afternoon threw the 3:50's back in it and stock tires to go back and forth to work.
get the RPM first and play from there.
Good luck and let us know where you turn,
Cavebull

I have to agree with GMC on the tuning side of this. I found my 55.2 with a Rochester model B to be jetted very rich. I have an air/fuel ratio meter I use for race car tuning primarily, it uses a wide band oxygen sensor and will measure ratios outside of stochiometric. Anyway, modern engines attempt to run as close as possible to 14.7:1 air/fuel ratio. My truck with the stock jet was at 11.1:1, very rich. No choke was needed to start it cold in 50 degree weather. I played with it for a while, unable to find a replacement jet and filled mine with solder and drilled it out smaller. I think I finally ended up at .040 bore size to get the ratio right. Driveability improved, throttle response was better and mileage increased.Food for thought.

Devin

Mileage and performance both good and bad, are a function of getting your engine and drive train in balance. Back in the day when gasoline was $.19 a gallon and our trucks were used for work and putzing along on roads that wouldn't support 65-70 mph speeds, the old trucks with their 8-12 mpg consumption, were still pretty economical. However, in their second or even third lives, and with 45-50 mph speeds constituting a "rolling road block" and gas prices nearing $4.00/gallon, regaining that "balance" can put you both up to speed, and give you surprisingly good mileage.

The key is improving on the efficiency of your engine and getting your drivetrain modified for the type of driving you most often you do with your "reborn" ride.

In a world that seems to be driven by the "horsepower game", I've discovered that it's really all about torque. And torque was what these engines were designed and geared to produce. When we take on a resto project we somehow focus on getting all the visual stuff upgraded, but are then disappointed by the fuel mileage. To complete a restoration, which you intend to put on the modern highway, you should also address all the issues that will effect performance.

A common misconception is that dropping your highway rpms will improve fuel mileage, but in reality, it's matching your gearing to your engines torque band and the normal speed you drive. An engine that is lugging at the lower end of the torque band isn't going to provide good fuel economy.

I've based my current fuel consumption (18-20 mpg) on my 54 truck over the past 5000 miles, on getting my 235 tuned and geared to match my particular driving needs. As 55Tractor points out, getting the 14-1 fuel/air ratio is tough with the stock carb and add to that, trying to run the engine at highway speeds with stock gearing, is only going to adversely effect engine performance and longevity.

I've discovered that after a vast number of engine modifications and using a 261 grind cam, the 235 torque band is STILL between 1600-2400 rpms. So based on this observation, I geared accordingly. In my particular case the T5 .72OD, 3.42 R&P with a 28" tall rear tire has produced fairly good performance and fuel mileage. I shot for 2000 rpms at 70 mph. However, I've found that my vacuum readings at a steady speed, will increase as I increase my engine rpms towards 2400. I've also discovered that most of my casual driving is well below 70 mph and that the legal speed limit and those that enforce it, frown on exceeding that limit. So, in my particular case, I need to jump up on my R&P ratio to 3.55 or in my case to 3.73. (Plenty of Camaro 62" wide 3.73 diffs) I seriously doubt that I will gain that much on fuel economy, especially with the E10 blended fuel, but my performance at my now more conservative cruising speed (60-65 mph) will improve or at least hold it's own.

The key is to match your gearing to your driving habits and focus on staying at 1900-2300rpms. That gives you a little cushion below and above should the need arise. It's really amazing what a couple hundred rpms can do to help or hurt your engine efficiency/fuel mileage. And with the reduced mileage from the E10(up to 25%) and the higher cost of fuel, it might be a good time to start paying attention to improving fuel economy.

So folks, crunch the numbers, gear so that you are closer to the top of the torque curve (1900-2400) at your normal cruise speed and you'll be able to enjoy your trucks great performance and save a few bucks at the pump.

Dave

Richie and all,
Dave nice addition to the post.
My 46 is set up with: a 3:90 rear , T-5 with a .72OD, 215/85/R16 Goodrich Commercial TA's (30.3"tall), stock 235 with two Rochester 2's set up progressive (dumper may only come in on the next motor, for now maybe never)on Clifford 4brrl intake, fentons, flowmaster 2 in 2 out muffler with 2 inch pipes. By the numbers should run 70mph, in OD at 2219 RPM, and in 4th (1:1)about 3040 RPM.

Questions:
1) Agree torque rules. Can one gain/get back some torque with the addition of a 1" carb spacer (not because it was wanted but out of necessity)?
2) Numbers at this point are theory, we are driving big shoe box's. How does the aero affect the numbers?

On a sad note, today is the two year purchase aniversary of the old my 46. I thought I would have been done. Again,what numbers get crunched may be adversly affected by the real road.

Cavebull

Crunching your numbers and comparing them with my actual driving performance numbers I'd be willing to say if the majority of your cruising was going to be from 55-70mph you're about as close as you can get with your current combination. It would have you running 2044 rpms at 65 mph and 2201 at 70 and based on my own experience these would be good solid performance numbers as far as gearing.

The amount of torque you could pick up by adding a spacer would be minimal, about like adding an additional carb. Adding dual carbs is not going to be a "kick-in-the-pants" type improvement, but you're going to get better fuel distribution and actually improve your overall fuel mileage if you drive sensibly. A lot depends on your dual carb selection and getting a carb that can be fine tuned and will hold that tune.

As for the aero on your "Shoebox" I wouldn't loose any sleep over trying to make improvements. With suspension upgrades, good rubber, sway bars front and back and decent disc brakes the AD will click along at 85mph even with the straight axle. Granted, it's not something you'd do comfortably for miles at end, but the 235 will easily reach that speed with the upgraded gearing. Of course you're going to be reaching the top of the torque range and your fuel economy will suffer.

And suffer it will with the E10. In addition to a 5-25% decrease in fuel mileage, the blended fuel is going to cause some major issues in our older engines performance. The ethanol blend is going to scour your fuel tank so an additional filter is really necessary between the tank and the pump. All rubber fuel lines are going to need to be upgraded to the blue-lined fuel injection grade hose. All replacemnt parts (fuel pumps)after 1985 are SUPPOSED to be E10 tolerant but I added a supplimentary electric pump at the tank to deal with "phase separation" which is a condition similar to old school vapor lock. If your engine was tuned for straight gas, you're going to have to richen your idle mixture significantly to eliminate a lean-burn condition and the tell-tale popping noted in the exhaust system during deceleration. The only up-side of this mandated blend is that it will help those folks out who can't get their carbs leaned down, and the internals of the engine will have less carbon buildup.

As for the validity of the numbers....if you back them up with actual time on the road and some documented observations you'll be far ahead of the EPA fuel ratings that we've been fed for years. I've taken the time to do just that and am more than willing to share my observations.

Dave

Richie, When I had my 235 in my 3/4 ton GMC with 4.56 gears, I was averaging 16 mpg at 65 mph. I picked up about 1 mpg and quite a bit of torque, power, and a good 500 rpm or more by getting rid of the Rochester 1bbl and replacing it with a 2 bbl Holley/Webber carb from Langdons.

OK I'm breathing.. I reading and re-reading.. I gotta take this all in and talk with my mechanic/re-end guy..
This is some great info..

So, I'm still researching and digesting all this great info on this thread.. Going over with my mechanic and it is apparent i have some more research and homework to do..
But i guess my question now is, How much improvement in gas mileage can I expect to see with any or all of the mods discussed on this thread? And, is it worth dumping the money into?
I still don't know exactly what the expected or average gas milelage was suppose to be for my particular model stock. I really wanted to start there.. Most people I have talked with that have had my set up have told me they were getting between 15 and 20 stock.. If I'm getting 10-11, that tells me that i have to first figure out why it isnt getting what it is suppose to be getting stock. At least thats my thinking.. Anyone wanna shed some light for me on this? Would be appreciated..

I know tune-up has been mentioned but, have you made sure your vacuum advance is working?
The other thing that has a big effect on gas millage with a Rochester carb is the base gasket. Most of the gaskets you get block the power valve port (good for 3 or 4 mpg).

intersting 68 Dave, is this a Roach 1 barrel your talking about?

intersting 68 Dave, is this a Roach 1 barrel your talking about?

Start with your driving habits. How/when/where do you use this truck? Is it driven short distances/2-10 miles/or long distances?

Are you an aggressive driver, or is there an egg between your foot and the throttle pedal? Are you the first or last off of a stop light?

I have done some tuning work on my pastor's old 71 ford wagon with 351 in it. He drives 2 miles to work every day. He can't get more than 9 mpg out of it. He accelerated pretty hard off of each stop, and te choke doesn't kick off until he kicks it down to shut it off.

I am getting 18 mpg highway out of a BB 427 race engine, putting out in excess of 500 hp with 4.56 gears and no overdrive. It can be done.

Devin

Trying to find the engine specifications in this 235.. Anyone have a tip on where to look? I've been google-ing but no luck.. Obviously I'm looking in the wrong places,,

By the way Devin.. I do about 50/50 freeway and city driving.. About 50 miles a day.. I drive slow.. never take it up over 60 miles per hour.. I try to refrain from zipping around in it.. I usually just cruise like an old lady..

Oh I think it's worth it. Here's what I'd do. Pull your plugs and have a look first. Black and sooty? Sooty black smoke from the tail pipe? My guess is that your Rochester is running filthy rich. That and your timing might be severely retarded. Once you resolve those, I think you should come up to at least 15 mpg.

And yes, I'd say the $140-ish expense to swap to the Holley/Webber carb was well worth it. Also, the $74 swap to a Pertronix electronic ignition conversion on the distributor was worth it too. Shoot, with $4+ a gallon gas, spending a few hundred bucks to modernize just a little bit and pick up 6 mpg should pay itself off pretty quickly.

Hot Rod Dad,
I think my intake manifold gasket is leaking and I'm loosing a lot of compression. Need to check it out..

And yes, I'd say the $140-ish expense to swap to the Holley/Webber carb was well worth it. Also, the $74 swap to a Pertronix electronic ignition conversion on the distributor was worth it too. Shoot, with $4+ a gallon gas, spending a few hundred bucks to modernize just a little bit and pick up 6 mpg should pay itself off pretty quickly. [/quote]

"I am getting 18 mpg highway out of a BB 427 race engine, putting out in excess of 500 hp with 4.56 gears and no overdrive. It can be done."

Holy crap I just saw a pig flying over head...sorry...had to say it.

Hey guys.. Fixed the manifold leak.. Threw in better spark plugs and took the oil bath off and upgraded to a better air filter system for the single barrel Rochester.. Definitely running heap loads better.. But my gas mileage still sucks..

Anyway, decided for sure to go ahead with a higher ring and pinion for starters.. The OD is something I will consider later.. From what I understand, I can get a whole new rear end for cheaper than taking apart the one I have and rebuilding it. I was told that those rear ends were all the same up to 1970 or something like that. And that they will bolt right up.. I'm hoping this is true.. But thought I would put it out to someone here who might have experience with say, swapping out an early 60's rear with a 1970 rear end.. I guess the idea there is to save a couple bucks, get a newer rear end and be able to find replacement parts easier.. I'm not sure if I have the 12 or 10 bolt.. Have to look tomorrow.. But I have to loose the 4.11 R&P.. It's way to low for my use..

So I'm floating around with the idea of a 3.55 or even as high as a 3.08.. My only concern is it dogging to much with the 3.08..? I haul nothing more than light ladders and paint cans.. I also have a small tool box bolted in the bed.. It actually sits inside the bed so you can't even see it unless you walk up on the truck.. At most it weighs 200 pounds when filled with my tools and gear..
So there you have it.. Plan "A".. Any input would be appreciated..

Richie Blue,

You asked about Chevy engine specs....I've found the following sight to be helpful and fairly accurate, with one exception....all 235 standard engine distributor rotation is clockwise. Check out their info at:

http://www.secondchancegarage.com/public/468.cfm

Also, you want to be very careful about crunching your numbers before you go swapping rear ends. Use this page to juggle your numbers until you're going to be at your normal cruise at about 2100 rpms....the 235 with basically a stockish cam is going to be happiest, with the highest vacuum reading at a steady speed, around that rpm when at cruise. That gives you a couple of hundred rpms above and below that number where your engine will be operating in it's optimum performance range. I like this site:

http://csgnetwork.com/multirpmcalc.html

Remember, my practical experience has shown that the 235 makes good power and torque in a range of 1700-2400 with the sweetest spots between 1900-2300. Cruising below or above that range is just going to be cutting into performance and fuel economy.

Dave

Dave, So cool, numbers put you in the market for gears. I would just make one more look and make sure the tire heght is factored 100% out by crunching the numbers. Tire heghts are more varried than Ring and Pinion combos out there. 10 bolt punkins will swap up to 62 for sure, maybe 63. Look in mid 50 ranges, Chevy cars and trucks. My guess would be a car (BelAire or caddy) with a auto trans powerglide will be more in the higher range. Trucks are 3:90 and 4:11 all day long. When I head to the junk yard I take a tractor jack and friend with me. Rare to find a R&P tag on rear now a days so will need to spin tire and count the number times the pinion turns - this is the ratio. I full turn of tire to pinion. I found a silver Sharpie work ideal fro marking both. Jack up driver side so tire, wheel, hub whatever are free to turn rotate. Climb under and count the number of turns on pinion/drive shaft. The friend is there to totate the tire and help pull you out if the jack slips or sinks in mud.
True story dude, the voice of experience - I was laying under a BelAire and it was so freaking muddy when the jack sank, the car's drive shaft pinned me down. If it were not for mud it would have crushed me the rib cage. They jacked the car up and had to dig around me in a couple of places to break the vacuum under me enough so I could roll out. Sorry, funny now.
Anyway, what is the tire height on your truck right now?
I have pictures of a punkin swap on the Fliker link below. Actually it is the whole mess but shows spider gear retainer bolt, c clip removal, axel pull out, punkin swap, and how it goes back together.
Keep us informed on the swap,
Cavebull

Cavbull and Fla54chey31.. You guys are cool beans.. Thanks.. I current have what I believe to be the standard stock 15" steelies for wheels.. As I said, this vehicle had two owners before me.. The first owner had the truck for years and the guy I bought it from only had it a few months.. He had two so he wanted to make room for something else.. Anyway, my point being is that from what I gathered from the seller and my research on line and my mechanic, the entire truck was basically un-bastardized and pretty much original..
I also went out and looked at my pumkin.. It's a 10 bolt.. I found this link that tells me all the rear end info is stamped on the inside of the pumkin on the passenger side looking toward the engine: http://www.348-409.com/differential.html
I haven't got under there to look but I will.. My rear end guy said for sure I'm running the 4:11 now..
I'm gonna go to those links and check it out..
I'm still sort of confused about the later model rear ends bolting up? It sounded like you were saying i can go older but not newer? Did I understand that right?
I'll re-read it..

Yuo are correct, look for a 10 bolt 63' back to 54'. The 10 bolts are open rears and bolt right in, even the spider gear and axles have same spline, thrfore same lug pattern and numbers, thus same wheels bolt up.
Make sure we are on the same page here, for your calcs, not the wheel/rim size (15" or 16") - you need the true tire height, from ground to top of tire for height wheel. Put a plumbers level across width of tread and measure from ground to bottom of level, this is true ride heght of tire. This number is the number put into the formula. Like we said, could be just a matter of a differnt tire size. 50's have shorter wall and thus short tires, up to 60, 70, 75, 85's.
Have you measured tire height this way?
Cavebull

you can crunch the tire height and ring and pinion on the is site www.randysringandpinion.com ron

Cool, thanks.. I went under the truck and searched the pumpkin and axle covers all around everywhere.. The only numbers I could find where actually located on the pumpkin itself.. (on the inside passenger side looking toward the engine compartment) They were stamped and read: MA 121 4
Don't know what that means.. Been looking on line to find out but so far no luck..

By the way., Cavebull, I have 75r15's on there.. The 25 inch

Ouch. If you drive the poor beast at 60, with a 25 inch tall tire, (per previous posts), and assuming a final 1:1 transmision ratio, , your clicking around 3600 RPM's...far from the "...happiest..." RPM given by dave of 2100. The only way to improve milage with this set up is leave it parked! No amount of tuning tips and changed driving habits is going to help this scenario.
To keep the same tires, transmission, and lope around at 60 in the optimum range you need a ring and pinion ratio of 2.4. Good luck no matter what year.

Me, I would fall back on a new set of tall tire first, a 30" tall tire would get you down to about 3000 rpm @ 60.
Next I'd look for the 3.0 range for a R&P to drop you to about 2200 RPM. It is my guess that high a ratio in a 10 bolt is going to be hard to come by, if not impossible.
Look at the cacultor ( http://www.randysringandpinion.com by run) really close and pick your poison.
Good luck
Cavebull

I hear ya cavebull.. But I have a tack installed on there and with my current set up I'm not doing 3600 rpm's at 60 mph.. I'm doing between 2900-3000 at 60 miles per hour.. Also, my rear end guy hooked up his own electronic tack and double checked everything.. So the tack I have is accurate.. Perhaps your math is off?
Also, on the Ecklers classic chevy trucks website I found a rear end conversion kit for $159 bucks.. According to this I can convert to a 12 bolt pumpkin that has a 3:08 R&P if I wanted..

http://www.ecklerstrucks.com/product.asp?pf%5Fid=121061&dept%5Fid=1861

Check it out.. I'm hoping this might be the answer.. Tell me what you think?

Anyway, my rear end guy told me that if I decide to go with a higher R&P that I should keep the size tires I have.. That's the only reason why I haven't stepped up the tire size yet..

Math is math - assumptions, such as 1:1 final trans ratio, calc tools, not seeing actual formulas and calcs, rounding errors, acurate numbers for variables, multiple variables, etc. is what throws the product off.
What do I think?
I think I missed by a blind calc by 600 rpm, you trust your rearend guy, and have come up with some options - time to make a desicion, sign a check, an let your mechanic start getting dirty. Let us know how it works out for ya.
Cavebull

Mr. Cavebull.. Actually, your math may not be off.. I just found out what MA 1214 means.. MA stands for 3:90 gear ratio and the 1214 supposedly stand for the month and day it was built..
So that would knock your calculations off slightly if you were thinking this was a 4:10.. Now my rear end guy didn't actually open up the pumpkin to look inside but he did mark the tires and roll it to count the rotations.. He came up with the 4:10 which is pretty close.. But I am more inclined to believe it is a 3:90 based on your number crunching and the RPM's I am currently getting at 60 mph.. and the fact that it has "MA" stamped on it.. Still, the 3:90 or 4:10 or whatever, is still way to low for my blood.. I'd like to try a 3:08 if I could swap out my 10 bolt for a 12.. It seems only the 12 bolts had the 3:08's in them.. I'm working on trying to find the answers.. One thing for sure I have noticed about rear end information, it seems to be an elusive thing to many and not an exact science.. Definitely everyone has opinions stretching the spectrum.. I'm actually quite amazed it is so difficult to get strait answers.. Perhaps part of it is that everyone has different needs and expectations and experiences.. So my challenge is to sift through and try to make an educated guess on the poison that best suits me.
I ran into a guy that said he was the go to guy for any and all parts that these Chevy/GMC trucks needed in the early 80's.. He said people ordered through him on the eastern seaboard and that there was another company out on the west coast called Goldcoast or something to that nature.. And that everyone went through them on the western seaboard.. Anyway, he said that he owned dozens of these old chevys with all the different engines from 60-66.. He is the one that first told me that all those rear ends swapped from 60-72.. Then he showed me pictures of a 1970 rear end he put in a 63 c10..
I'm still doing homework and I really appreciate this site.. all those links very helpful..

oh, and I'm trying to find out what my final gear ratio is on my manual 4 speed (1st is a granny gear) transmission so I can accurately calculate what R&P poison to install... Any tips on where I may be able to scare up that number?

Again, go to the link provided by We B Old or formula provided by GMCdp.
http://www.ringpinion.com/Calc_RPM.aspx

The nuumbers do jive at 3.9 and RPM is at about 3K. There is a lot of flex in 100's of RPM's. Not exact science.
With the numbers as close as they are I would be inclined to agree with GMCdp, that your final ratio for the tranny is 1:1 in 4th, pretty much a standard unless OD'd.
Everyone has opinion and the biggest differnce is Stovebolt guys are not making money or getting kickbacks. Be cautiious out there greasshopper!
I think a lot of us on the site know this and would agree - the final decision is yours to live by as it is your truck, your ride, and the way you role. Go back to the begining of the thread, everything you need has been addressed.
1) Drive conservative. ((Devin 55 tractor)
2) Make sure the truck is in proper tune. (GMCdp and Devine 55 tractor)
3) Find optimum RPM for HP and torque. (Dave Fla54Chevy3100 Fla54Chevy3100)
4) Do mthe math. Add in tire height, gears ratios (rear and tranny) to place your driving conditions in the "motors sweet spot" as much of the time as possible.(Dave Fla54Chevy3100)
5) get the optimum R&P.
6) Fine tune with tires.
Eessential there is no more except your followup on choices and outcome.
good luck,
Cavebull