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Old 05-10-2002, 07:00   #1 (permalink)
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One for the Maths and Stats people

Ive got a pretty stock EB2 XR8 with 225/65 15 inch wheels, auto and the stock 3.27:1 diff. Lets say I got this car up to 4050rpm in top gear, what sort of speed would I be doing?

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Old 05-10-2002, 07:13   #2 (permalink)
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Formula is thus:

MPH=(RPM x Wheel Diameter (in inches)) divided by (Gear Ratio x 336).

(BTW the wheel diameter is not the rim diameter) - it is the wheel and tyre.

In the example quoted you would be travelling about 98.6 mph
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Old 05-10-2002, 07:19   #3 (permalink)
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does that take into account its got an overdrive 4th???? The bastard does about 116km at 2000rpm (and the cops confirmed this) so less than 160km/h at 4000rpm doesnt really work for me. Because the speedo was pointing seriously southward.
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Old 05-10-2002, 07:50   #4 (permalink)
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Oh and if you want the formula to calculate wheel diameter it is -

2*pi*(wheel rim/2+(aspect ratio*tire width/25.4))

.. wheel rim is in inches and tyre width in millimetres - the result is the wheel diameter in inches.
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Old 05-10-2002, 08:23   #5 (permalink)
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Old 05-10-2002, 16:07   #6 (permalink)
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One rotation of rear wheel = 2.115m

4050rpm Engine Revs = 1821 Rear wheel revs (using 0.68 O/Drive, assuming auto)
4050rpm Engine Revs = 1720 Rear wheel revs (using 0.72 O/Drive, assuming manual)

KM/Hr = 231 for Auto, 218 for manual.
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Old 05-11-2002, 08:36   #7 (permalink)
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Wheel Diameter in Inches = 2X((Section Width X Aspect Ratio)/2540) + Rim Diameter

eg for 225/75R15 tyre we have x 2(225X75)/2540)+15
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Old 05-11-2002, 20:55   #8 (permalink)
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Are you people taking into account wind resistance and driveline loss?
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Old 05-11-2002, 22:34   #9 (permalink)
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Quote:
Originally posted by Falchoon
Are you people taking into account wind resistance and driveline loss?
It's a good point you raise, not sure what the speed drop could be, but this would only affect an auto wouldn't it, I'd imagine the torque convertor would be unable to lock up at that speed, so that would drop speed by 20~30km/hr, however a manual shouldn't loose speed from this unless the clutch was slipping.
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Old 05-12-2002, 05:35   #10 (permalink)
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Unless you have major wheel spin or similaar driveline splippage I don't think wind resistance and driveline power loss will change the realtionship between rpm and mph just your ability to get to the max. Of course if you are going fast enough the tyre diameter will also increase slightly.
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