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[Formulae to find or verify speedo, tach & diff ratio]


  • Given: 5280 feet per mile, X 12"= 63360 inches per mile
  • I use "%" to indicate "divide by"
  • Find wheel diameter using a level on the top of tire

To find RPM using speedo reading: Wheel Diameter X Pi = Wheel Circumference (26 X 3.14= 81.64") Inches per Mile % WC = Rear Wheel Rev per Mile (63360 % 81.64 = 776) RWR/Mile X Rear End Ratio = Engine Rev per Mile (776 X 3.08 = 2390.36) ER/Mile X Miles per Hour = Engine Rev per Hour (2390 X 75 = 179277) ER/H % 60 Minutes per Hour = Engine Rev per Minute @ given MPH (179277 % 60 = 2988 RPM @ 75 MPH)

To find any RPM at any given trans ratio, multiply final RPM times that ratio: 200-4R overdrive of .76: (2988 RPM X .76 = 2271 RPM @ 75 MPH)

To find rear end ratio using tach & speedo: Engine RPM @ Given MPH X 60 minutes = ERPM per Hour (2988 X 60 = 179280) ERPHour % MPH = Engine Rev per Mile (179280 % 75 = 2390) ERPMile % Rear Wheel Rev per Mile = Rear End Ratio (2390 % 776 = 3.08)

To verify speedo by mile markers & stopwatch: Feet per Mile % Elapsed Seconds per Mile = Feet per Second (5280 % 30 = 176') FPS X 60 Seconds/ Minute X 60 Minutes/ Hour = Feet per Hour (176 X 60 X 60 = 633600) FPH % 5280 = Miles per Hour (633600 % 5280 = 120 MPH)

Example: 5280 % 55 seconds X 60 X 60 % 5280 = 65.45 MPH

Take your time, write 'em out, it's not that hard. John

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