Picking an axle ratio is usually done backward from how you want the vehicle to feel at cruising speed: pick a target RPM, and work out what ring-and-pinion set gets you there. This calculator does exactly that — solving the standard gearing formula for the axle ratio instead of the RPM, then snapping the answer to a real off-the-shelf ratio and showing what each nearby option does to your cruising RPM.
Why solve backward from RPM instead of forward from a ratio
Most gearing calculators go one direction: pick a ratio, and see what RPM it produces. That's useful once you already own an axle, but it's the wrong tool for choosing a re-gear. This calculator inverts the relationship — you supply the RPM you want at a given speed, and it tells you the axle ratio needed to get there, using axle ratio = (target RPM x tire diameter) / (speed x 336 x transmission ratio). The 336 constant is the same unit-conversion factor used in forward RPM calculations; it just moves to the other side of the equation.
Why the exact answer rarely matches a real gearset
Ring-and-pinion sets are manufactured in a limited number of standard ratios — this calculator checks against the 8 most common: 3.08, 3.23, 3.42, 3.55, 3.73, 3.90, 4.10, and 4.56. Your exact computed ratio will almost never land precisely on one of these, so the Nearest Common Ratio tab snaps to the closest real option and shows how far off it is. A small difference (a few hundredths) barely changes cruise RPM; if you're choosing between two ratios that are both close, the Effect on Cruise RPM tab lets you compare the actual RPM each one produces at your speed before you buy gears.
Balancing target RPM against torque and economy
A lower target RPM generally calls for a numerically lower (taller) axle ratio — quieter, more fuel-efficient cruising, at the cost of weaker low-speed torque multiplication for acceleration and towing. A higher target RPM points to a numerically higher (shorter) axle, trading fuel economy and noise for more usable torque at the wheels. Tire diameter and the transmission's overdrive ratio both shift this trade-off: a taller tire or a deeper overdrive both lower RPM for any given axle ratio, effectively letting you run a shorter (higher numeric) axle while still keeping cruise RPM in check.