# Subject: Gear Ratios - MPH

Date: 13 Jul 1995 04:01:30 GMT

Math is not my expertise, so I kept it real simple...

### MPH =( A x B x pi x 60) / ( C x 12 x 5280 )

A = Engine R.P.M. (revolutions per minute)
B = Tire Height in inches (from ground to top of tire)
pi = 3.1416
60 = used to convert minutes to HOURS
C = Gear Ratio (Ring gear/pinion gear teeth)
12 = used to convert inches to feet
5280 = convert feet to miles per hour.

(To metrify, convert mm to inches for tire size, then convert
MPH to KM) Fellow engineers or math majors might provide a
reduced formula and one for our metrified netters...)

From Kraiger2@aol.com Fri Feb 2 15:46:07 1996
To convert mm to inch dimensions....Multiply the number of mm by .03937
Example: 20mm(20 x .03937=.79")
(This comes in handy in the belt business. Kraig)

EXAMPLE:

55.51mph=(1750rpm x 29.00in x 3.1416 x 60)/(2.72 x 12 x 5280)

29" is the approximate height of 235/75R15.

Using this formula helped me select the correct tire size for my '81 Jeep Wagoneer Diesel conversion... the Targetmaster Diesel will be turning the same RPM at 55mph as it did in my Oldsmobile!

--
John Meister
Snohomish, Washington - "where FULL-SIZE JEEPS don't rust, they mold..."