Gear Ratio & RPM Calculator
Bigger tires change everything downstream of the engine. Enter your old and new sizes — see the RPM drop, the gearing you're effectively running now, and the re-gear that brings it back.
265/70R17 — 31.6" tall
35X12.5R20 — 35.0" tall
Cruise RPM at highway speeds
| Setup | 55 mph | 65 mph | 70 mph | 75 mph |
|---|---|---|---|---|
| 31.6" tire · 3.73 | 1,527 | 1,805 | 1,944 | 2,083 |
| 35.0" tire · 3.73 | 1,379 | 1,630 | 1,755 | 1,881 |
| 35.0" tire · 4.10 re-gear | 1,516 | 1,792 | 1,929 | 2,067 |
Top-gear (overdrive) ratio varies by transmission — check yours; 0.70 is a typical overdrive, 1.00 means direct drive. Speedometer and odometer will also read off after a size change — see exactly how much →
Going bigger? We stock the tires.
33s, 35s and 37s in all-terrain, rugged-terrain and mud-terrain — wholesale pricing by the set, pallet or container.
Common questions
Why does my truck feel slow after bigger tires?
Bigger tires effectively raise your gearing: a 3.73 axle with 35s on a truck that came with 31s behaves like roughly a 3.30. The engine turns fewer RPM per mile, so acceleration, towing and sometimes fuel economy in hills all suffer until you re-gear.
What gear ratio do I need after going to 35s?
Multiply your current ratio by (new diameter ÷ old diameter). From a 31-inch tire to a 35 on 3.73 gears: 3.73 × 35/31 ≈ 4.21, so most people pick 4.10 for highway bias or 4.30–4.56 for towing and off-road. The calculator does this for your exact sizes.
How do I find my axle gear ratio?
Check the door-jamb or glove-box axle code, the tag or stamp on the differential cover, or count driveshaft turns per wheel revolution (with the other wheel locked). Your dealer can also decode it from the VIN.
What is the top-gear (overdrive) ratio?
The transmission's highest-gear ratio. Overdrives are below 1.00 (0.70 is typical); direct-drive top gear is 1.00. It multiplies with the axle ratio to set cruise RPM.