Gear Ratio / RPM / MPH Calculator

What this number means

This is the question under half of all regear threads: 'what will I turn at 70?' The answer is pure arithmetic once you know top gear, axle ratio and true tire diameter. The table also shows one gear down — because the real question is usually whether the transmission will hunt between gears on a grade, and that depends on the gap between the top two ratios.

What to change next

If cruise RPM lands under ~1,800 on a gas V6, the rig will downshift on every overpass. Over ~2,800 and you're paying in noise and fuel. Most dual-purpose rigs are happiest geared so 70 mph sits between 2,100 and 2,600 RPM in top gear.

Worked example

A JL Sport (850RE top gear 0.67, 3.45 axle, 31.5-inch tires) turns about 1,555 RPM at 70. On 35s with no regear it drops to ~1,400 — deep in the lug-and-downshift zone. 4.56 gears on those 35s bring it back to ~1,850 with crawl to spare. Run your own numbers above.