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Old 06-21-2013, 11:36 PM   #21

Scooter11's Avatar
Drives: 2013 SW 2LS
Join Date: Feb 2013
Location: Northern NJ
Posts: 872
Originally Posted by snymat68 View Post
These numbers are for my '13 1LS M6 with 18" wheels. Change out the bold numbers for your own and recalculate.

The math...

A = Vehicle Speed (in mph)
B = Wheel Radius (in inches) measured from the wheel center to the ground
C = Rear Differential Ratio (ie 3.27)
D = Transmission Gear Ratio (ie .84)

VEHICLE - Convert MPH to in/min
(80mi/1hr) x (5280ft/1mi) x (12in/1ft) x (1hr/60min) = 84,480 in/min

WHEEL - Convert radius to circumference
C = 2πr = (2) x (π) x (13.75in) = 86.39 in/rev

WHEEL - Convert in/rev to RPM
(84,480in/1rev) x (1rev/86.39in) = 977.89 RPM (at the wheel)

RPMs - Work it backwards through the drivetrain
(Wheel RPM) x (Differential Ratio) x (Transmission Gear Ratio) = Engine RPM
(977.89) x (3.27) x (.84) = 2,686 RPM
Wow, that's really useful! It reminds me of AP Physics though
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