The formula is not logarithmic. In fact, it couldn't be any more linear.
The proportional example is a fine way to work the problem, if you have
a tach hooked up. Otherwise, a good formula to use is (mph x gear ratio
x 336)/ tire diameter
tire diameter is in inches
A TH700-R4 has a final drive of .70:1. To figure RPM with one of these,
multiply your rear gear ratio by .70.
For those who care for theory, 336 is a constant with the dimensions rpm
x in x hr per mile. You could do a dimensional analysis and compute this
number.
I used a Chrysler 8 3/4 inch rearend on my truck-- very strong and hella
cheap. I paid 100 for the entire rear. It came with 2.93 gears, which
were no fun with a 700 overdrive. I got 3.55 gears, and the truck is
much happier now. Engine turns 2087 rpm at 70 mph.
http://www.mit.edu/~kdurand
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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