The Stovebolt Site Forums The Shop Area Driveline Gear ratio vs. 235

Hi all, I have a '53 3100. It has a stock '56 235 in it with an open yoke SM420 and a 3.50 rear gear. It cruises very nice and I don't have any issues with highway driving. My math tells me that I'm turning about 2400-2600 rpm at typical highway speeds, and it definitely sounds like it. I would like to change to a 3.25 or maybe 3.00 final gear in order to drop the cruising rpm's a bit. If I change to one of these gears, will it become a problem with starting out or maintaining highway speed? There are no major hills where I live. Does anyone have experience with this situation? Thanks

I have a 235 in my 1952 and 3.55 gears. I also run pretty tall 6.50 x 16 tires. I wouldn't want to gear it any higher than it is and will switch to 6.00 x 16 tires if I ever wear these out. I do have hills and it will maintain speed but takes a lot of throttle to do so. I think I would have trouble towing even a light weight boat. I wish there was something more like a 3.73 gear set for my stock rear end.

Granny in the sm420 should still pull a house even with a 3.00 gear. I don't use granny much at all. My main concern is being able to maintain highway speed without dogging the snot out of the engine.

Tim thanks for the advice about the 3.90 gears although I suspect they may be hard to find in good shape. I have the SM420 trans. I am actually pretty happy with the 3.55's except for fantasizing about pulling a boat or an old canned ham trailer with it. Realistically, the stock brakes and the huge jump in insurance costs will probably keep me from ever doing that. (It currently has very reasonable classic vehicle insurance).

Yes, you will be extremely disappointed with the performance. You're running approximately 1,000 RPM below the speed where the engine is HP rated, and just slightly above the torque peak- - - -ideal speed for good performance and hill pulling or hauling a lightweight trailer. Install a clutch-type fan or maybe a flex fan that won't make so much noise at cruise, and let the engine wind up. You're probably hearing fan noise, not the actual engine, and it will run all day at 2500-3000 with no damage. Dropping the cruise RPM as low as you're suggesting would be a huge mistake!
Jerry

There seems to be a lot of concern about over-revving these engines. Although I am sure it can be done if you were a lunatic, it just isn't an issue, given the low horse power/low max RPM that these engines produce under even a minor load.
2400-2600 RPM at highway speed is right at the optimal power curve for your engine.
I will bet that you have plenty of gas pedal left in reserve at highway speed at 2400-2600 RPM.
If you change anything, you will lose that.
With a granny first gear and the 3.55, you have the best setup of all for a stock torque tube truck.
The only setup better than that for a 235 would be a T-5 transmission (w/overdrive) and a 3.73 rear axle (open driveshaft).
Carl

Not that it matters, but I do have an open driveshaft in this truck.

The truck would be a slug and have challenges getting out of its own way.

I was curious on this topic too. Would the T-5 and 3.73 rear gear be ideal for modern highway speeds without excessive wear on the engine?

Los59,
I sent you a PM.

I agree 100%. I run mine around all the time at 3k+ and it runs just fine. Doesn't get hot and doesn't even use that much gas in my opinion.

My story on gearing. Have a 37 Buick with a 52 straight 8 that has 9 more HP and 20 more #/ft of torque than a 235 but also weighs a thousand pounds more so things are about evened out. I wanted to be around 2000rpm at 60-65mph so stuck in a set of 2.73 gears to go with the 350 automatic adapted to the straight eight. A torque converter is actually a torque multiplier so in the flat lands of Texas it was WONDERFUL but then I got into the hills of Missouri and it SUCKED. Glean whatever you want from this story.

Now, i need to put my 2 cents in, I put a 56 235 in my 46 1/2 ton with a T-5 and a s-10 rear with 308 gears and 235-75-15 in tires and I have no issues at all with RPM or speedometer, I pull a teardrop trailer on the interstate and back roads and hills like im in a new modern truck, of course certain size hills,im gonna have to downshift, but im not climbing huge mountains either ......

Hoozat, what is the gear ratio for 1st gear of your T-5? (They are available in different ratios.) That is a crucial consideration when running a 3.08 rear end so that the truck can get out of its own way on a hill or when hauling a load.
Carl

I'll jump in. Sorry Hoozat, if I'm stealing your answer.
The S10 T5 comes with either a 4.03 or a 3.75 first gear. The 1964-66 trucks with a column 3 speed usually had a 3.73 rear and a 2.95 first gear. So 2.95 X 3.73 = 11.00.

I've always heard that the combination of first X the rear should be somewhere between 9 and 12. So 11.00 worked great for these trucks.

So 4.03 X 3.08 = 12.41 and 3.75 X 3.08 = 11.55. Both of those results are really acceptable. Folks with the 3.73 or 3.90 rears usually feel that the S10 first gear is more like a granny gear. Having the 3.08 solves that.

Second third and fourth gears are simply incremental steps getting you up to speed.
So then fourth gear would be 1.0 X 3.08 and have lower RPMs than the factory tranny. That's good enough but the extra 5th gear sweetens the pot if you happen to have the 0.86. Fifth gear of 0.86 is what I recommend for a 3.08 rear since 0.86 X 3.08 = 2.65. The 0.72 OD gear would make a low HP truck lug. You want the OD gear X the rear end somewhere between 2.7 or 2.9 ideally.

I had a 1935 Buick model 46, three window coupe,
straight 8, 231, 93 HP, with a 4.11 rear.
I did not do this all the time but wanted to find
out how well a straight 8 could pull.
I could slow down to 12 MPH in 3rd and the car
would not shudder when I accelerated.

If I would of wanted to modify my old Buick and it
was possible I would of went with the stock rear
but add a 4 sp or 5 sp manual. Or better yet a new
Chevy/GM 6 sp auto with two OD gears.

What we could do if we had the money. Building things
to see if they worked out as good as we thought.
Jeffrey

I'm surprised nobody has mentioned vacuum yet on this thread. One common misconception out there is that lower RPMs are always better for cruising/economy. That is not necessarily true. Ideally you want to run your engine at highest possible vacuum for the speed you are driving. The engine runs most efficiently at its highest vacuum. I started a thread a few years ago called "driving with a vacuum gauge." There was a lot of great info in that post. I'm not savvy enough to post a link to it in this post.

Someone posted the analogy of pedaling a 10 speed bike. At cruising speed you can run it in 5th gear all day long and not get tired. That is a good cruising RPM at the pedals. Shift it into 10th gear and your RPMs drop significantly. Problem is you have to pedal 3 times as hard to keep it at speed. After 10 minutes you are wrecked. You get a similar result what driving your stovebolt. You end up having to press down much farther on the throttle to keep at speed at a lower RPM. More throttle equals move fuel and less vacuum. I found this out when I installed a 3 speed overdrive in my 58 Suburban with 3.90 gears. I was all excited that I could drive around at 65 MPH at 2400 RPMs but couldn't figure out why my mileage didn't increase. I was getting the same MPG as before the overdrive install.

Anyway, I'm really rambling. Check out the vacuum gauge thread for some great info.

Edit. Found the thread but still can't post the link. Search for "How to drive with a vacuum gauge???" in the daily driver forum.

Larry,
I will quickly agree that the engine is performing best when vacuum is high.
I'm not a disagreeable person. I just like discussing this stuff since it educates my brain.

However, I don't think we actually drive that way. What I mean is, if we are on the freeway, we stay at speed with traffic. Or at least we try to. If we are out for a drive on a country road, we adjust speed according to the curves and road conditions. We usually stay right up near the speed limit if the truck handles well enough to do so.
Here is how I think. My 65 C10 with factory 3 speed would turn 2031 RPMs at 47 MPH. The RPMs would be 2809 at 65 MPH. I think we all would agree the truck would get better MPG at 47 MPH than at 65 MPH. Wind resistance would play a huge factor. I don't know what the vacuum gauge would show at either speed. The other side of the coin is to say, If the manifold vacuum was higher at 2809 RPMS, I doubt you'd agree that speeding up to 65 would save gas.

My real life example is this. My 1965 C10 can now travel at 65 MPH at the same RPMS that it had at 47 MPH. That should improve MPG since I still have a Rochester B. I might do a test. I might drive it and just use 4 gears and no overdrive. That would be just like the 3 speed with 1:1 in 4th gear. Then see if my MPG drops or improves.

Folks with different carbs may see different MPG changes because of the variables in how and when certain carb fuel circuits engage based on vacuum.

"How to drive with a vacuum gauge???"

When I was much younger, a "gas saver gauge" was commonly sold.

Tim,
I'm playing with you a little here. Was overdrive common when the "gas saver" gauge was popular? I just can't imagine that having a vacuum gauge would make me change my speed on the freeway, nor on a country road. If I was driving on a deserted highway and could choose my speed for optimum MPG, then I'd find it useful.

no problem lugnutz, I have the 4.03 in mine and have no hill climbing issues and drives on the interstate at around 1800-2000 rpm at 60-65 mph with my 235, like it was made that way...lol

Thanks for chiming back in with your 4.03 first gear ratio with your 3.08 rear end. Between your real world experience and Lugnutz math, the rest of us know what to select from the bone yards.
Carl

My math teacher would be proud.

Lugnutz;

My situation is similar to yours. With an overdrive trans I can now travel at 65mph with the same RPMs as about 45-7 in direct drive 3rd gear. The issue is that unless I'm cruising along on a flat road, it takes a lot more gas pedal to keep at 65mph. I found that if I didn't change my driving habits and simply drove along at 65mph in overdrive no matter what, I got the same mileage. I expected to see a significant jump in MPG after I installed the overdrive and it didn't happen. I ended up driving around with my foot in the carb a lot more thinking I was good to go because the RPMs were lower.

The main point I'm trying to make is that being in overdrive all the time isn't necessarily the most economical. There are times (when traffic slows or going up hill) that you will see better economy by getting out of overdrive and back into direct drive. Wind resistance also plays a big part. Pushing my brick of a truck down the road 10-15mph faster requires a lot more fuel as well.

Bottom line is there isn't always a direct correlation between reduced RPMs and increased mileage. How you drive, what speed you maintain and when you shift into and out of overdrive all play a big part.

Yes driving with a vacuum gage is sort of a must
do option when adding overdrive transmissions and
or putting in lower gear ratio rear ends. For many
times it will be better to downshift rather then
to open the throttle more. Jeffrey

Well, after reading all the differing posts, which I truly appreciate, I ended up finding a set of 3.25 final gears for free. So I took out my 3.50 gears. I had to pull an axle anyways to repair a leaking axle seal. cleaned up the 3.25's, put in a new pinion seal, and swapped them out. (F@rd 9" by the way) I figure the best thing to do is to try the change and judge for myself, for my situation, with my setup, on my road conditions, with my driving style. It is an extremely cheap and easy trial, so why not. Now I only have to wait for the summer to show up. With several feet of snow on the ground and temps in the -30F degrees below zero, I just can't see myself going for a test spin!