Gear Ratio & Tire Size Chart
Good "rule of thumb" calculation is to mutiply .12 by your tire diameter. (.12 X 38" = 4.56)
(see more formulas at bottom of page)
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 slightly higher, and the opposite is true if you have overdrive (your RPMs will be slightly lower).
Common gear formulas:
Gear Ratio
Crawl Ratio
P-Metric to Inch Diameter
Tire Change (find new gear ratio)
Speedometer Adjuster (with oversize tires)
(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH
Simple Gear Ratio Formula
Ring Gear Teeth Count / Pinion Gear Teeth = Gear Ratio
thanks for sharing