The Stovebolt Site Forums The Shop Area Driveline What rear end will work for a 235 straight six

Need some advice I’m getting ready to swap out my rear end closed drive line, I picked up a Chevy 10 bolt rear end unknown truck year, gear ratio according to be 3.08-3.25
Width from drum to front drum 65 1/2 stock rear end mesures 621/4 front to front of drum. I’m also going with a 3 speed manual transmission

I have a 1979 nova but it has a 2.73 ratio but I am sure there are others. The springs are wider. Mine is hooked up to a T5 from an s10.

What diameter tires are you using?
A 3 speed transmission with a 3.08 rear end with 28" tires will be nice on the highway. It will not be nice with a load on a hill in first gear, nor with one tire in a hole. Your clutch will slip like crazy while you try to milk it to keep the engine from stalling. I speak from specific experience.
Shorter tires will help this situation, but it will cost you on on highway speeds at a reasonable RPM.
As they say, no free lunch.
Carl

What speed do you want to drive on the highway? A 235 has max torque at 2000rpm so gear it to run a 2000 at the cruising speed you want. Anything above or under this engine speed will cost you at the gas pump. Some think (wrongly) that the fewer the rpm the better the economy. If an engine is constantly in a high demand low vacuum mode it costs you.

How would you do that calculation? I came up with 2.77 to do 60 MPH @ 2,000 RPM. I’m thinking my calculation may not be correct. If it is, I don’t think that is going to work especially starting up a hill.

I’m looking to go at least 60-65 mph so guys I don’t plan on RACING in a 235 engine straight six, I’m going to be crushing with no load in back of the bed for now.

Your calculation seems about right. The Camaros with the 6 cyl 250 came with a 2.73 rear axle, same as Ron's Nova mentioned above. First gear in the Camaro was 3.50:1

Sorry guys I forgot to mention I will like to stay with a 6 lug pattern.

Calculators.. I have 235 75 15 on the rear and can go as fast as I want and still not exceed 2000 rpm. I live in flat land Indiana and it works well here. But I go to Virginia it does ok on interstate still ok but on back roads sometimes it requires some shifting downshifting to 4th. Around town, it will keep up with all but the boy racers at stoplight drag. BTW it is a 273 gear.

Does anybody know the gear ratios for a AD three speed, in a 1/2 ton. 52’ to be specific.

I believe it to be 4:11.

That would be the rear end gear. What I was referring to were the ratios for each of the gears. Third is 1 to 1, but what is first and second?

http://chevy.oldcarmanualproject.com/chevyresto/52152.html

First 2.94 : 1
Second 1.68 : 1
Third 1 : 1
Reverse 2.94 : 1

I've read you want a factor of 9 to 9.5 for street machines for first gear.
Using above ratios, 9.5/2.94 = 3.23 rear axle ratio.
Tire diameter plays a factor as well.

For 60's Saginaw 3 Speed:

# of shaft grooves 1st Gear 2nd Gear 3rd Gear

0 2.85 1.68 1.0

1 2.54 1.50 1.0

2 3.11 1.84 1.0

3 3.50 1.89 1.0

reverse is 2.95

A 2.73 rear axle with 2.85 first gear = ride clutch to get moving.

Thanks buoymaker, I just got my 52’ 3100 3 speed w/235, 4.11 rear on the road and at 45 MPH she is so noisy I can see some driveline change her the future. For now I need to work out all the other issues that attribute to the noise first before I make that determination. Looking at the 4 speed on the floor, using second gear to start, with a 3.58 ratio, it seems to be a better choice for lowering you gear ratio that the 3-speed.

Phak1 has moved this thread into a new configuration.
What year 235 (low pressure or high pressure engine) and what application (car/truck 235)?

3.55 R&P swap and retain the torque-tube (close to the 3.58 you mentions)

SM420 4sp transmission

To remove possible noise-makers, replace u-joint and all seals/bushings/bearings (buy the kit).

Thanks Tim, my 235 is a 59’ from a car with hydraulic lifters, 3 “on the tree” transmission with the original 4:11 torque tube. It seems to me that with a lower ratio on a SM420 (3.58), it would be a better transmission for changing to 3.55 or higher rear gears than the 3-speed with a starting (1st gear) ratio of 2.94.

I have safety issues to deal with first, wandering steering (another post) inadequate brakes, (already looked at rears, need to look at front), and seat belts (feel naked without them). Got to tackle those first. At this point, I can’t even imagine this truck going any faster than 45 MPH, and at that speed I felt unsafe.

And I agree that I need to fix what I have prior to making improvements. So, I’m really just looking at future options. Who knows, I may like what I already have.

"It seems to me that with a lower ratio on a SM420 (3.58), it would be a better transmission for changing to 3.55 or higher rear gears than the 3-speed with a starting (1st gear) ratio of 2.94. "

FYI: I have a similar set-up (and, I like it). I normally use 2nd gear as my 1st gear. (unless pulling tree stumps, or, competing in a slowest-truck race). Even with the 3.55 rear, that SM420's 1st gear is really low.

A rear end from a 1969 Chevy 1/2 ton pickup will fit nicely. You can find these with 3.07 gears. Match that with a T5 transmission with these gears: 4.03 for first, 2.37 for second, 1.49 for third, 1.00 for fourth and .86 for fifth.

Your original 3 speed and a 4.11 rear end will produce these final ratios: 2.94 x 4.11 = 12.08 for first gear. 1.68 x 4.11 = 6.90 for second gear and 1.00 times 4.11 = 4.11 for third. Let's say those are your benchmarks.

Now please watch what happens with the T5 I mentioned and a 3.07 (or 3.08) rear end: 4.03 x 3.07 = 12.37 for first gear. 2.37 x 3.07 = 7.27 for second gear. 1.49 x 3.07 = 4.57 for third gear. 1.00 x 3.07 = 3.07 for fourth gear and .86 x 3.07 = 2.64 for fifth gear. Please look at the comparison...your first 3 speeds are going to be close to the original ratios in the factory 3 speed. What this means quite honestly is that you can still put 1/2 ton of stuff in the bed of your pickup and drive off without burning up your clutch. In other words, your pickup will still drive like it was designed. Your original tires were 6 x 16 and had a 28 inch diameter. If you have slightly larger diameter tires (29 inch, for example) you can multiply each of those final T5/3.07 ratios by .97 (making them 11.99, 7.05, 4.43, 2.98 and 2.56. If you increase the diameter of the back tires please be mindful of your caster angle...you will reduce it unless your front tires are also 29 inches in diameter and reducing that angle will make your truck wander around. At 29 inches diameter some other things in your front end change (scrub point for one) but we're not going into those with this post. Steering geometry is another topic altogether.

Here is the bottom line and the point I'm trying to make: The ratios produced by this transmission and rear end are one way to have your cake and eat it. Your truck will feel just like it did when it was built. Then when you shift to 4th, you'll have a 25% reduction over the 4.11 rear end...this will be your first overdrive gear. And when you shift to fifth, you have your second overdrive. If you're in a very hilly area, you have the option of driving in 4th gear because 5th isn't really that much of a reduction. In my opinion this is a great scheme to solve the gearing challenges in a stock AD truck on today's roads.

Please also pay attention to what Evan (Coilover) has told you. 2000 rpm is your target. With the combination above and a 28 inch diameter tire, you will be turning 2066 at 65 mph. Driving a stock AD truck faster than 65 isn't a brilliant plan. The front end and the brakes simply were not made for speed. Reduce your speed to 63 and you'll be at 2002 rpm. You won't get closer to your target than that. If you have the right carburetor or if your carburetor is in very good condition, you'll be getting the best gas mileage you can ever hope for and your engine won't be lugging. You can use other rear end and transmission ratios to mimic what I've done here, but please sit down with a cup of tea and do your own calculations and please be mindful of the final gear ratios. A high speed rear end mated with a stock transmission in an AD pickup is going to mess your in-town driving up like crazy and you won't be able to haul around a picnic basket in it. If you don't believe what I'm saying about final drive ratios, please go to Google and look up something like Starting Line Ratio. Forget anything you read about a great SLR for a 400 horsepower engine. You don't have one of those. Drill down until you find a great SLR for a six-cylinder pickup. Good luck!

I am running 3.55 in the rear behind a 216 with 3 speed. I can cruise at 70 mph all day long with rpm level of about 3200-3400ish.

I know nothing about the optimum horsepower for balancing maximum torque and fuel efficiency.

However, if I am reading this 1959 235 truck diagram correctly, maximum horsepower is achieved at about 3200 RPM on this diagram (max torque at about 1600 rom). And, the torque vs horsepower cross-over RPM is around 2600 RPM.

I can run all day at 60-65 mph in my 1954 Suburban (2500 RPM). I did it recently for an 8 hour drive to Macungie, PA. All "original equipment" front-end and brake equipment is in excellent condition; and, the truck has highway-speed radial truck tires on original wheels.

I am inclined to side with Tim in that the optimum RPM is closer to 3,000 than 2,000 (2,600 probably is on the money). You won't ever hear the engine roar at around 2000 RPM, but you will not get the gas mileage you think that you should. The problem is the carburetor. As you approach the slightest incline and step on the gas to keep your speed, the power valve will puke fuel into the venturi in a clumsy and less than precise manner. This is what kills gas mileage for these engines with these carburetors.
Modern vehicles get away with ridiculously low RPM by controlling spark timing and fuel delivery with a computer. That is how they get such good fuel mileage.
I run a T-5 with .72 overdrive and a 4.64 rear end gear with 28" tires. I can pull stumps in first gear and drag them down the road at 60 MPH. The 235 isn't screaming, but the RPM is in the somewhere between 2,800 and 2,900 at 60 MPH.
I have some 215 85 R16 tires to put on it very soon (30.5 inches). This will make first gear a little more useful starting from a stop (a little on the granny side with the 28 inch tires), and reduce my RPM at 60 MPH (hoping for 2,600 to 2,700). There is a handy table to get the exact figures that I am shooting for. I lost my bookmark for it, so I am going off memory. The seat of my pants are indicating that I should be pretty close.
Carl

Yes, one of the main problems is in the carburetor. That's why I said "If you have the right carburetor." Last summer, I posted the following paragraph about the Rochester B/BC carburetors. I was (and still am) looking for possible ways to reduce the vacuum loss at the power piston...something "everyman" could do easily. Rochester wanted the piston to stay off the valve at 9 inches of vacuum or greater. If it would do that, there's a comfortable margin. The ones I looked at didn't and couldn't...at 11 to 13 inches the spring would push that power valve wide open. If you need more vacuum than 13 inches to keep the power valve out of play, I don't think there's any way that carburetor is going to give you good gas mileage unless you're driving on downhill roads with the wind at your back. And if your carburetor is running that rich, it doesn't matter what rpm you're turning. Efficiency is gone.

Here's what I posted...

The wear of time and use causes these carburetors to run more richly. Why? Couple of reasons. The constant loss of vacuum through the worn power piston bore will be between 2 inches at the best and probably more like 3 or 4 inches. For all I know it could be more in some cases...I can only judge from the ones I have here. This lost vacuum prevents the piston from working as designed. It was designed to drop down and cause the power valve to activate in Rochester's own words "at approx. 9" hg." When it was new, I'm certain it did. Today with the wear I've observed and testing with vacuum, that piston could drop down at anywhere between 11" and 13" and that puts the power valve into play at least 20% sooner than it was designed. The flip side...with the wasted vacuum, your power piston will require more vacuum to lift it back up and off the power valve. And the more the carburetor wears, the worse this will get. The o-ring approach gets us very close to the 9" Rochester envisioned...actually a bit less (which is on the good side if you're hoping for a leaner mixture).

I don't believe total gear ratios are determined by multiplying the transmission gear ratios times the rear end ratio but by adding the ratios.
IE: 2.94 x 4.11 = 12.08 should be 2.94 + 4.11 = 7.05.
Fred

Well, let's think about it this way: in fourth gear which is 1 to 1 (meaning one engine turn equals one driveshaft turn) you would add 1 to the rear end ratio of (say for example 3.07) and this would give you an overall ratio of 4.07? How about in fifth gear where 86/100 of one engine turn equals 1 driveshaft turn? That is to say in fifth the engine is turning more slowly than the driveshaft at this point by about 14%. If we added .86 to 3.07, we would have 3.93. Seems like multiplying them makes more sense, doesn't it?

Incorrect Fred, you do not add the trans gear ratio to the rear end ratio, you multiply just like Jon says.

It would help if Ernie could give us some more information. Reading his other previous posts, I get the feeling he has a well used original truck and wants to put it on the road without sinking a ton of money into it.

1.) Getting back to Ernie's statement, replacing a 62 1/4" (drum face to drum face) stock rear axle with a 65 1/2" (truck year unknown), will this fit? That's 1 1/2" on each side. I can't answer that question. Ernie are you going to go back with stock wheels and tires? Also, a 10-bolt Chevy rear axle should have an axle code somewhere. Wire brush the grease and dirt off and look for a code. This may help you identify the year and hopefully the gear ratio. There is a lot of Chevy 10-bolt rear axle information that can be found on the Internet as well as how to relocate the spring perches.
2.) Swapping out the original closed drive and going with open drive 3 speed transmission, Ernie which 3 speed transmission (manufacturer and year) do you plan on using?

For the time being I’m staying with stock wheels which are 15” rims tire size 560
As for the Gm 10 bolt rear axle I currently have, I don’t no if I’m researching correctly on the website with the #’s I found are Gm32 140055251 C215 26067927
For the 3 speed transmission I have now is open drive out of a 55 Chevy Bel Air car 3 speed transmission no overdrive, The only difference on the stock transmission is that the shift lever is pointing up and the transmission out of the 55 bel air is pointing down.

A quick look and I could find no info on those numbers either.

https://www.crankshaftcoalition.com/wiki/Wheelbase,_track_width,_and_differential_measurements
The GM Truck section shows years 73-87 and 73-91 with the second dimensions of 65.8" axle flange to axle flange.
Your rear axle could be from a C10, K10, C20, K20 etc. of those years.
I would try posting (have to register first) at www.67-72chevytrucks.com to see if they can help with the identification.
They have a section for 1973-87 trucks.

Yes, I saw your post for the car transmission. Looks like you got some good advice on how to make it work.

I counted the teeth on the rear end I have counted 41 teeth now dividing by 10 comes out to 4.10 gear ratio correct me if I’m wrong. Thanks

41 ring gear and 10 pinion gear? Yes.
http://www.demandaam.com/technical-support/aam-technical-tips/to-determine-gear-ratio

Your original closed rear axle ratio would be 4:11 for 1/2 ton (3100) truck.

hi i was reading about ratio here and read you still have original wheels . here my problem , maybe you you have suggestion .
i have 1952 gmc three quater ton with 16 inch split rims. i would love to get rid of split rims . with there age they scare me. what are your rims . and do you know what i should but to stay at right diameter wheel an solid rims, thanks you for your time

The problem with 16" drop center rims is that they probably won't fit over the brake drums. A 16" single piece spoke type "wagon wheel" is sold for trailers that has the right lug bolt pattern, but be sure to check for brake drum clearance before committing to buy them. You might try to find a 17.5" tubeless tire rim with an 8-bolt center. It will have tires that have the same overall height as a 16" tube type tire. The downside is that 17.5" tires are pretty spendy.

There is absolutely nothing dangerous about running multi-piece rims as long as they're not rusty or damaged. The only dangerous ones out there are the Firestone RH-5 rims that split in the middle, and those were never used on 3/4 ton vehicles, to my knowledge!
Jerry