Thread: 20+mpg small block with carb.

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2.73 times the .7 overdrive will produce a final rear gear ratio of 1.91
3.42 times the .7 overdrive will produce a final rear gear ratio of 2.40
3.73 times the .7 overdrive will produce a final rear gear ratio of 2.61
4.11 times the .7 overdrive will produce a final rear gear ratio of 2.88
The spread on the 700R4 is so dramatic that with the 2.73 gear you'd have a nearly unuseable 4th gear and with the 4.11 gear you'd have a nearly unuseable 1st gear. 3.42 or 3.73 would seem to be a good compromise, or use a 2.73/3.08 rear gear with a TH350.
Rear tire diameter will make a difference also.

One useful number I came across during review of the 700R4 transmission is that the combination of rear gear and tire size (look up rev/mile) needs to be such that the engine is turning 2000 rpm or more in the 4th OD gear at 70 mph. I can just make the rpm at 2023 rpm with a 3.55 Ford 8" rear and large P235R/75R15 tires (698 rev/mile). Since almost every dyno chart you see starts at 2500 rpm you need to worry a bit over whether the car body is sufficiently streamlined at 70 mph and that the engine has enough torque to move the car in 0.7 final OD gear at 70 mph. As Tech said the best rear ratio might be in the 3.4-3.7 range and my setup might better have been a 3.7. The formula for rpm is

(70mph)(tire rev/mile)(0.7)(rear ratio)/(60min/hour) = rpm in OD at 70 mph

I am using a stock stall converter at 1650 rpm so that is another worry, the stall rpm has to be less than 2000 rpm as well.

Don Shillady
Retired Scientist Teen Rodder

Originally Posted by Don Shillady

I am using a stock stall converter at 1650 rpm so that is another worry, the stall rpm has to be less than 2000 rpm as well.

Don, that's the STALL rpm for the converter. It will only slip at that rpm when you have the loud pedal to the mat in DRIVE with the brakes locked. If the converter slipped at that rpm all through the range, millions of grandmas would be at their dealerships, raising hell about engine noise on the way to bingo.

OK Tech, can you estimate at what low rpm the converter will slip based on a 1650 stall rpm in the OD gear? Normally the trans will downshift before this happens so this rpm determines the shift point?

Don Shillady
Retired Scientist/Teen Rodder

OK this post is supposed to answer a general set of questions by 1972Chevy regarding setting up a SBC for higher mpg. While we have Tech1 available for discussion maybe I can learn something and help 1972Chevy at the same time? The STALL rpm has to do with how high the rpm can go against a locked no-go situation but I am not aware of a rpm rating for when a locked-in gear selection will either slip at the converter or stall the engine. I am sorry to say my brain was frozen in the 1950s relative to a "road test" site near my home. It was/is a short (about 100 yards) steep hill (about 40% grade) approached by a sharp left curve so you could not accelerate much at the bottom before attempting the grade. The "test" was would your car be able to reach the top of the hill in high gear and many a Model A could make it due to the heavy flywheel and low rpm torque. Later model 59AB flatheads would barely make it and the engine would "buck" and almost stall near the top of the hill unless the clutch would slip. Today most automatic transmissions just downshift on that hill and few notice the problem except that the hill is steep. So with a 700R4 in OD locked in either electrically or by an internal hydraulic condition about what rpm would the transmission downshift? A 305 SBC with a Crower 000902 cam suggested by Tech1 is a good idea but the question is whether the shift point for the OD will be low enough to shift into OD at say 55 mph where higher mileage is desireable? I have sweated this out trying to match tire size to my 3.55 rear ratio and 700R4. My engine (350 sbc) now runs but I have yet to set the shift points. A local transmission guy says my 3.55 rear and 235/75R15 tires are in a good range with less wheelspin than with a 3.73 rear but still good traction and potentially reasonable mileage with the 700R4 shifting into OD somewhere around 55 mph but I have been checking the rpm values at various shift points and it looks close to me whether the shift point into OD will be low enough to help the mpg. I will only know after the trans shop sets the shift points for me. Perhaps my worrying and the formula I have provided will help 1972Chevy? At the very least 1972Chevy might want to worry over tire size as well as the rear ratio.

Don Shillady
Retired Scientist/teen rodder