The Stovebolt Site Forums The Parking Lot '60-'66 Trucks Need opinion's for better Revs

I recently purchased a 65 c20 with I6 and the casting number on the engine says its the 230. My tranny is a 4 speed with the granny first Not sure what rear axle gears I have but they feel like 373 or 411's and i currently have 15 inch wheels.

Currently, going 55mph the truck is buzzin very high rpms.

What would be the best and or cheapest option to make? I'm only looking to go 70 max and that would be on a rare occasion.
I'm going to use the truck as a cruiser only. Quick take off is not necessary either.

Can you guys give me options and maybe what vehicle's I would want to search for to make it happen?

Everyone's advice is welcome!
Thanks ahead of time!

Bigger rear diameter tires will help some, more than you would think.

See this chart: http://www.sierrabronco.com/TireGearRatio.htm


I suggest jacking up the rear of your truck and figuring out just what gear ratio you have:

Pulling the rear differential cover (& counting the teeth) is the most accurate method to determine gear ratio but another way to check the rear end ratio is to jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers -or- 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

Once you know what you have you can make an educated decision on what to do!

I hope this helps!

If it's still the original rearend, most likely you have 4.56 gears. There really isn't a cheap fix for getting the rpm down, there's really only 3 ways to do it. 1, bigger tires as Hot Rod John says. Of course, this will get into changing wheels to be able to get a large enough tire with the right load rating, most likely a 16", plus the cost of tires. 2, overdrive transmission. Again, can get pricey depending on what trans you use. Another way with the trans is an add on overdrive, like the Ranger Overdrive from Advance Adapters or a Gear Vendors, but they are very pricey. 3rd option, swap the rearend gear. Unfortunatly, there really is only 1 option if you keep the stock differential, and that's the '68-'72 4.11, which was found mostly in V8 automatic trucks. High demand/low availabilty is the hangup there. You could also swap the entire rearend if you're not concerned with keeping the truck original, there's plenty of late model rearends for 3/4 ton with 3.73 gearing, this would be the cheapest way to go.

I went the cheap route and just live with it. I switched from 235/75R/15's to 265/75R/15 Rear Tires on my '72 C-10. It did make a difference, but I still avoid the Interstate. In your case, I would recommend going to 16 inch wheels, as mentioned above, and get just about the tallest tire you can find. But you will then probably always need to start off in Granny 1st. I always use Granny in all my trucks, but it seems like many don't. Heck, if I had a 10-Speed, I'd use all 10, lol.

Bill

I do believe it is the original rear end so its probably 4.56s. The tires I have now have an overall diameter of 27 inches. According to the link HOT ROD gave me that would have me in 4th gear with RPMs at 3689.

I would like to land somewhere in the blue or green range by this diagram: http://www.sierrabronco.com/TireGearRatio.htm

In my opinion 4.56s are not necessary for me and keeping it original is not an issue either.

Longbox suggests I could easily go with 3.73s, is it possible for 3.21 3.31 3.42 3.55s or would that be unfeasible to do?

It depends on what carrier you have. Pull the rear cover and get the facts and then you will know what the best plan of attack is.

If you have a number 3 carrier you can go all the way down to 3.08 gears...

Here is some more data for you from: http://richmondgear.com/07Apdfs/page27.pdf

GM 8.875 12 bolt rear ring and pinion data.

RATIO PART NO. TEETH CARRIER
3.08 49-0068-1 40-13 3
3.42 49-0070-1 41-12 3
3.73 49-0072-1 41-11 3
3.73 49-0280-1 41-11 4
4.10 69-0204-1 41-10 4
4.10 69-0350-1 41-10 3
4.56 69-0206-1 41-9 4
4.56 69-0352-1 41-9 3
4.88 69-0298-1 39-8 4
5.13 69-0300-1 41-8 4
5.38 69-0302-1 43-8 4

Didn't the 70's & 80's Buicks have 272 rearends running the big block 455's? I think the 5-lug wheels on them were the same as 5-lug Chevy trucks too. You'd still have to do some fab work to adapt the axle and your wheels are probably 6 lug...but, to really cruise effortlessly without a lot of power...a 272 or 307/08 would really be the way to go. Probably gets gobbs better mileage too. That's my 2 cents...Jim

Too low a rear end ratio and that poor little 230 will have a hard time pushing the truck in 4th gear. You'll end up driving around in 3rd or 2nd! lol!

cm

NV3500 Getrag would work well in that application. Used in Pickups, and still retains a 27% overdrive. Bolts right in, just a little xmember work and speedo.

Hot Rod John. Looking at the info that you have posted, I assume that those part numbers on your chart are for Richmond gears only? I have a factory 4:11 Posi in our 66 Burb, and I want to put in a 3 series set of gears. I see that they list 4:11 gears in both a 4 series and 3 series carrier. From the factory, is it possible to have a 3 series carrier with 4:11 gears? And if so, how do I confirm which I have?
Thank you for your help.

My information states that for stock trucks using the GM 8.875" 12 Bolt Differential:

3-Series: (3.42 and numerically lower)
4-Series: (3.73 and numerically higher)

Richmond Gear as well as many other aftermarket gear makers have redesigned the wheel, so to say, and make gears such that if you know which carrier you have, their gears will fit it.

I hope this helps.

We've gotten a bit off topic here. buzzindozens truck is a C20 3/4 ton, so none of the 1/2 ton stuff will work. Just from my quick research on the most likely rearends that would be used for a swap, the GM 14 bolt, the Spicer 60, and the Spicer 70, it looks like 3.73 is the common gear. There are listing for a 3.42 in the 14 bolt, and a 3.54 in both the 60 and the 70. I do know that the 60 is pretty common in the 3.54 gear, usually in Dodge or ford van applications.

I have a 64GMC 3/4 ton with a dana 60 rear end.It had 411 gears in it when I got it, I had 353 put in and it drive much better at hwy speeds now.to find what kind of rear end you have look in a summit catalog,there you will see a differential case reference guide.Then take the cover off and the gearing should be stamped into the ring gear.

I keep seeing this question asked so I thought I would provide a way to answer the question, What rear end and what transmission do I need. Here is the way to figure it out........

Engine rpm X 60 divided by
tire revs per mile X axle ratio X transmision ratio
Equals MPH
2000 X60= 120,000 div by
696.57 X 3.73 X .75 = 61.58MPH with a 700r4 in 4th gear at 2000 rpm

If you want to find out which rear axle ratio you need to run at a given rpm with a 700r4 here is the formula:

Engine rpm X 60 divided by
tire/rev/mile X mph X trans ratio
If you want to run 70mph with a 700r4 at 2000 rpm you need a 3:37 to 1 rear gear

to find out the revs per mile of your tires, take one and mark the bottom of the tire with a piece of chalk. roll it across the floor until the white line comes around. measure this distance convert it to feet and divide 5280 feet to get the rvs per mile . In mycase with a 265/75/15 that was 696.57 revs per mile. Kieth

700R4 final ratio = .75 4 speed, 3 speed = 1.0

If you change tire sizes you can see the outcome on a given ratio, trans, etc.

I do not think you can use a 3 series gear with a 4 carrier. I was looking for a posi trac unit and have 3:73 gears, they had to go to the 4 series carrier for this ratio. Do not know about the Richmond gears. Call here 918-425-7725 Axle transmission and transfers ask for Richard, he can give you the low down. Kieth

That's the advantage of going aftermarket. Richmond makes higher numerical ratio gears that fit into the 3 series carrier. You can go all the way up to a 4.56 gear in a 3 series carrier. Notice on this link how they have the steeper gears for a 3 series carrier: http://www.richmondgear.com/07Apdfs/page27.pdf

Thanks, I was able to get a rebuilt posi unit (4 series carrier) to go with my 3:73 for $250.00 so this worked out well for me. Kieth

I did a little homework and tried to develop a chart that would show how the different transmissions ratios 1:1 -- 1.25:1, tire sizes, rear axle ratios, and tire sizes come together to see how changes effect the drivabilaty of a combination. I did this in the mid eighties for Class 8 trucks and it really helped pick the correct combination of trans, engine, rear ends, and tire sizes when the speed limits were changing, along with newer diesal engines, wierd overdrive ratios, different tire sizes etc. The speed limits were changing from 55 to 65 then to 70mph while the tires were going from 11:00 24.5 to Lo pro 22.5. It was kind of like shooting at a moving target and to top it all off fuel prices were going up so dropping rpms at a given speed was very important.
I set out to find out what kind of power we had been getting to the ground for known combinations of drivetrain, and then using a new set of charts and tables what the changes we were making were doing to our crusing road speed and available torque.
What we finally devised was a table that showed USABLE TORQUE to the ground and another table that shows road speed at different rpms for the same eng/trans/tires/Rear axle ratio. The results were interesting because one could change everything on the truck and see how it affected the operating rpm at different speeds, and how it was going to effect the usable power.

Here is a example: Torque within a usable range of speeds

I like to use 2000 rpm and 65mph as a reasonable fuel economy mode and which will still pull well around town. In this case the 3.42 with a 700r4 has 580 ft lbs of torque in OD and a 200hp 350. My truck now has 627ftlbs available in 3rd gear with a 130hp engine but it is having to turn too high a revs. A change to 3:42, 700r4, 200hp means it will have more power and better fuel economy. Kieth

265/75/15-----230-----250------292-------350------350
----------------120hp--130hp-----150hp-----200hp----250hp

3.08
3---or--4spd--478ft#--518ft#---597ft#----697ft#---871ft#

OD-1.25-------358ft#--388ft#---448ft#----523ft#---653ft#
____________________________________________________________

3.42

1:1-----------506------548------632-------736------921

1.25 to 1-----398------431------498-------580------725
____________________________________________________________

3.73

1:1-----------579------627------724-------979-------1059

1.25to 1------434------470------543-------734--------791
___________________________________________________________

4.11

1:1-----------638------691------925-------930-------1162

1.25 to 1-----478------518------694-------697--------872


After Looking at this chart which shows torque not HP, you can compare it to what you have now and other powertrain combos that you know work pretty well. None of this is supposed to represent actual numbers since no losses are taken into account but it does give one a way to compare these variables and have some idea how they will perform compared to other combinations. Kieth

Did I scare everybody off here ?

Nah, you're doing a fine job,I think?

Gee it seems like the easiest and cheapest way would go to a salvage yard dnd pick up something in the 350 range for about $50. I have done mine and enjoy the results. I wouldn't go any below 350. 378 might be a better choice.

Go look in the hi-pro section for more on this.