Re: What's the formula for figuring road speed w/4.88 gears.


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Posted by Charlie on February 19, 2000 at 17:21:24:

In Reply to: What's the formula for figuring road speed w/4.88 gears. posted by Jonas on February 19, 2000 at 15:56:47:

With 4.88s, if you've got the fender clearance and the power:
You could run 9.00R16s or 36X12.50R16 radial Swampers, about 576 rev/mile, for 2800 rpm at 60, 3270 at 70, 2570 at 55. Or perhaps you could find a 33X10.50R16.5 that would fit on you 6.75X16.5 rims, which turns at ~630 rev/mile, for 3070 rpm @ 60 mph. Your current tires turn about 680 rev/mile, or 3320 rpm at 60. with 4.88s and a V8 I think 36s would work OK, especially since you don't have an overdrive.
The formula is: rev/mile of tire * gear ratio = rpm @ 60 mph.
Rev/mile is NOT exactly 5280/pi*diameter, because of slippage and squirming. For reference, a 31.7" radial is 657 rev/mile, a 36.1" = 576; you can interpolate by for example:
rev/mile = (31.7"/D) * 657
If D = 32.8" (33X10.5R16), rev/mile = 31.7/32.8 * 657 = 634 (approx., did it in my head)
And 4.88 is actually 4.875, 4.89 is 4.888888888888etc.9

Charlie


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