What RPM will you turn at highway speed? What did that tire change really do to your gearing? Two quick calculators with the math the shops use.
Engine RPM = MPH × axle ratio × transmission gear ratio × 336 ÷ tire diameter in inches. The 336 folds the unit conversions (inches per mile, minutes per hour, pi) into one constant. It is the same formula printed in every gear vendor's catalog.
A bigger tire covers more ground per revolution, so the engine turns slower for the same speed, exactly as if you installed a numerically lower axle ratio. That is why a truck on 35s with stock gears feels lazy off the line and hunts for gears on grades.
The same percentage your effective ratio changed is the percentage your speedometer and odometer are wrong by, until the computer is corrected for the new tire size. On modern trucks that correction is a setting in a tuner or a dealer flash, not a gear swap in the dash.
It depends on your top gear ratio. With a 0.70 overdrive, about 1,860 RPM. In direct drive (1.00), about 2,660 RPM. Run your exact combo in the calculator above.
Multiply your current ratio by the new tire diameter and divide by the old tire diameter. A truck with 3.42 gears and 31 inch tires that moves to 35s needs about 3.86 to perform like stock, so a 3.90 or 4.10 is the practical choice since ratios only come in set steps.
It changes the RPM where top speed happens, not the truck's aerodynamic or power limits. Deeper gears usually make a truck feel stronger everywhere at the cost of higher cruise RPM and fuel use. The calculator shows the trade exactly.
Pistons & Ink is a digital product studio based in Ocala, Florida, building web and mobile products for the automotive enthusiast community. We come from the custom car and truck world — lowriders, minitrucks, C10s, restomods — and we build tools we'd use ourselves.
This gear ratio calculator is one of 11 free tools in the Pistons & Ink Digital Toolbox, built for the aftermarket auto industry. If it helped you, the rest of the toolbox is right here.
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