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Assuming you are in 5th gear...................... (archive)

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Posted by John A on February 24, 2002 at 17:37:05:

In Reply to: What diff have I got if.... posted by Geoff on February 24, 2002 at 14:09:34:

5th has a .81 ratio, so the driveshaft is turning 4800 / .81 = 5926 rpm
To figure this, you also need to know the rolling radius of the rear wheels.
I measured my 225/45 X 16 tire at a rolling radius of 11 5/8".
feet/rev = 11.625 X 2 X pi / 12 in/ft = 6.09
Convert mph to ft/min:
100 mph X 5280 ft/mi / 60 min/hr = 8800 ft/min
rpm of the axle = 8800 ft/min / 6.09 ft/rev = 1445 rpm
The axle ratio is the rpm of the driveshaft divided by the rpm of the axle:
5926 rpm / 1445 rpm = 4.10 (stock US M3 axle ratio)
YMMV, depending on tire size, tire wear, or inflation pressure.
Incidentally, at the published top speed of 143 mph, engine rpm is 6864.
John
88 ///M3 Cinnabar

If I am doing 4800 rpm at 100 mph? I think it equates to 19 mph per 1000 rpm - and being thick I don't know how to calculate what diff I have. E30 M3 BTW

Thanks
Geoff





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