Running a 235 chevy at lower RPMs

Those old motors were designed to rur with some steep gears and I have one now with tall gears,it seemed to get better gas mileage with the 4.10 gears then with the 2.79s I have in it now so what can be done to make them run more efficient with taller gears. What I have is a 57 235 in my 37 chevy p/u and I took out the original 4.10 gears and 3 speed and replaced them with a 8 inch and a T-5,I know I should go with gears in the 3.40 range but one would think by not spinning the motor over like it did with the 4.10s would get better mileage but it did not.

My '54 Bel Aire had a 4.11 '55 rear and overdrive, it got 18 in high and about 13 in OD. Now my '52 235 stick gets under 20 with the stock 4.11's. I'm going to put in 3.55's to get a bit more cruising speed, but I'm not expecting any mileage gain. Those old Stovebolts just ain't got it in 'em!

I guess they were designed in the days of 20 cents a gallon for gas and no highways but I do want to see if I can get the mileage back up some,I am going to step the gears back up to around 3.50 and I forgot I drilled the main jet in the carb a little so I will install the jet from a parts carb and see what happens. With the 4.10s and the fenton headers it was running a little lean so I drilled the jet out a little but with the talller gears now it might need the smaller jet back in,when I put the T-5 and modern rear end in I was not expecting any more gas mileage but the drop in mileage suprised me.

FWIW powerglide cars had 3.55's and stick cars had 4.11's... mialage reported on the powerglides was the same as reported on the stick cars.. this is information I found in the old car manual project.. estimated 18 MPG

Just reread your original post, you have a 2.79 rear and a T5? That's just a bit over 2:1 overall ratio, not really surprising the Stovebolt doesn't like it! Try running it in High, not OD.

It does not go into 5th unless I am going down hill or on a very flat road abouve 60,I do plan on some gears that are better suited for the motor but I was suprised its drinking gas like it is just running around town. It might not be really using much more since the speedo is not correct but is seems like it and I cant get the speedo corrected untill I get the gears I want to use. I was wondering if a cam with a different profile like a RV grind would help it be more effficient now that the motor is not spinning as fast as it was.

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Originally Posted by junkyardjeff

It does not go into 5th unless I am going down hill or on a very flat road abouve 60,I do plan on some gears that are better suited for the motor but I was suprised its drinking gas like it is just running around town. It might not be really using much more since the speedo is not correct but is seems like it and I cant get the speedo corrected untill I get the gears I want to use. I was wondering if a cam with a different profile like a RV grind would help it be more effficient now that the motor is not spinning as fast as it was.

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Crower still does a lot of cams that are considered mileage grinds and will do custom regrinds also.I would talk to there tech people and give the info on your combo and see what they can do for you,back in the 70's during the Jimmy Carter "gas crisis" some of the RV mileage grinds they made would improve 4-5 MPG with the right parts.

I put a stock sized jet back in the carb and will need to take it on a small trip and see how it does,the plugs looked like it was running a little rich and putting the taller gears has changed the fuel demand of the motor alot since when it had the 4.10s the plus were always burning white.

With a Rochester Vacu-jet carb that is stock equipment on a '57 235, it will have very little manifold vacuum at cruise with tall gears. I don't recall the spec's, but I'll bet the power enrichment valve is open on your carb at cruise speed, that causes high fuel consumption. You can experiment with different springs above the piston or snip a bit off a spare spring and see if that makes any difference. Remember, less spring tension delays the valve action, no spring at all defeats it entirely.
Put a vacuum guage on it and drive by it, keeping as high a reading as you can. I'll look in my books for the vacuum values for the carb valve, but I don't recall seeing them for years. Too many books!

As for your speedometer being wrong, S10 FWD's have nifty little gearboxes that go on the trans end of the cable and change the cable RPM. Any I have gear it up for lower ratios, though, may not be made to go the other way.

I will tinker with the power valve spring this week and see what happens.