Several people recently queried about how to
calculate either what the actual rpms or car speed
should be, given one or the other. Well, awhile back I
pondered the same thing as I had several tires sizes &
gear ratios to play with, and came up with a fairly
simple formula I'll share. Please bear in mind it is
absolutely linear; no tire growth, nor wind drag, nor
falling torque curves, but it does at least help you to
see what you got.
What I wanted was given tire size & gear ratio, to be
able to put in rpm & get out mph, or vice versa. This
is what I came up with, assuming a 1:1 top gear;
drive tire dia. in feet
rpm x (.0113636) x ----------------------- = mph
final gear ratio
or,
final gear ratio
mph x ----------------------- / (.0113636) = rpm
drive tire dia. in feet
For example, running a 3.91 rear end ratio and a
tire with a 5.68' diameter (185/60HR-13), at 6000 rpm;
5.68
6000(.0113636) x ---- = 99.05 mph
3.91
or,
3.91
99mph x ---- / (.0113636) = 5997 rpm
5.68
cheers-
dstone@sc9.intel.com
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