Calculators Crawl Ratio or RPM/MPH Tables Tire/Gear/RPM table |
GEAR
RATIO CALCULATOR
This calc can be used to find any missing value. .If your tires are bigger than stock you either guess at the actual MPH or do something like count the seconds between mile markers on the highway while maintaining 60 on the speedometer. If you know your gear ratios and tire size you can get a fairly close number using this calculating tool. Fill in all entries except the one you want an answer to in green section only. Formulas used (RPM X tire Diameter) |
Good "rule of thumb" calculation is to mutiply .12 by your tire diameter.
(.12 X 38" = 4.56)
The below table can be used to get a rough idea on gear ratios. The
colors represent ideal RPM's at highway speeds (65). For highway cruising and best fuel
economy stay towards the yellow (2600 rpm), around town daily driving is color coded green
(2800 rpm), and for better towing power or just more 4-low power use the ratios near the
red (3100 rpm). These calculations are assuming a manual transmission with a 1:1 ratio. If
you drive an automatic your RPMs will be higher, and the opposite is true if you have
overdrive (your RPMs will be lower).
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