Thread: 20+mpg small block with carb.

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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