Speedometer Conversion Constant?
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Speedometer Conversion Constant?
I just performed a speedometer cluster change from non tach to one with tach. I kept my original speedometer with the unchanged conversion constant from factory. Whenever I purchased the truck originally in late '92 I purchased 9.0 rims and installed 275/60R15 (from original 235/75R15 Firestones) and never changed the conversion constant until after I added the new tach. The constant was originally at 972 and from what the manufactors (Kelly, Michellin, Cooper, etc.) stated the rev/mile were (I took an average of all) and got 740 RPM then multiplied that by the conversion constant of 0.0135 as stated in the proceedures and got the new conversion constant of 9.99 (excluded the decimal). What I want to know is how the conversion constant multiplier was derrived (0.0135 for 15" rims / 0.015 for 16" rims).
