The one formula that ties it together
Engine RPM at a steady speed comes down to four things: how fast you're going, your axle ratio, the transmission gear you're in, and how tall your tires are. Put together:
RPM = MPH × Axle × Trans × 336 ÷ Tire Diameter
The 336 is just the constant that turns miles-per-hour and inches into revolutions-per-minute. Use the transmission ratio for the gear you actually cruise in: 1.00 for a direct-drive top gear, or something like 0.74 in overdrive.
Re-gearing after bigger tires: multiply your current axle ratio by new diameter ÷ old diameter. Went from 32" to 35" on a 3.55? That points you at about 3.88, and a 4.10 gives back a touch more grunt than stock.
Higher ratio or lower ratio?
| Axle ratio | Feel | Best for |
|---|---|---|
| Numerically higher (4.10, 4.56) | More torque, higher cruise RPM | Towing, big tires, hills, off-road |
| Stock (3.55, 3.73) | Balanced | Mixed daily driving |
| Numerically lower (3.08, 3.31) | Calm cruise, best MPG, lazy launch | Highway miles, light loads |
There's no free lunch: the ratio that tows and climbs best also spins faster and drinks a bit more fuel on the highway. Match the ratio to how you actually use the truck. Our truck gear ratio and axle ratio guide works through the trade-off with real numbers.
Changed tire size? Get your exact diameter first with the tire size calculator, then drop it in above to nail the re-gear.
