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...)




To convert mm to inch dimensions....Multiply the number of mm by .03937


Example:  20mm(20 x .03937=.79")






  
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!  


-- 


 


 
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