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.
Changed tire size? Get your exact diameter first with the tire size calculator, then drop it in above to nail the re-gear.