Speedometer difference
With a speedometer calibrated for the 235/75R15, this is your real speed on 31x10.50R15 tires.
| Speedometer shows | Actual speed with 31x10.50R15 | Difference |
|---|---|---|
| 20 mph | 21.5 mph | +1.5 mph |
| 30 mph | 32.2 mph | +2.2 mph |
| 40 mph | 42.9 mph | +2.9 mph |
| 50 mph | 53.7 mph | +3.7 mph |
| 60 mph | 64.4 mph | +4.4 mph |
| 70 mph | 75.1 mph | +5.1 mph |
| 80 mph | 85.9 mph | +5.9 mph |
Fitment considerations
- Width: the 31x10.50R15 is 1.25 in wider in section width, so each sidewall sits roughly 0.62 in further out on the same wheel. Check clearance at full steering lock against the upper control arm, frame and fender liner, and confirm your wheel width is inside the tire maker’s approved rim-width range for the 31x10.50R15.
- Height: 2.12 in of extra diameter means about 1.06 in less space between the tread and the wheel well. Depending on the vehicle this can call for a leveling kit, a lift or trimming; check with a tire shop or with owners of your exact model.
- Speedometer and gearing: a 7.3% diameter change is worth recalibrating for. The effective axle ratio changes by the same percentage; see what it does to your RPM with the gear ratio calculator.
- Ride and handling: a taller sidewall generally gives more cushioning and flex, with slightly softer steering response.
- Inch sizes are nominal: flotation sizes are often rounded, and the measured diameter of a specific tire model can be noticeably smaller than the number on the sidewall. Check the manufacturer’s specs.
- Load rating: match or exceed the load index (and LT load range, if applicable) on your door placard. P-metric and LT tires of the same size are rated differently.
These checks are general; whether a size fits a particular vehicle depends on its suspension, wheels and body. Your door placard lists the approved original sizes, and a tire professional can confirm fitment.
How these numbers are calculated
Diameter = 2 × section width × aspect ratio ÷ 100 ÷ 25.4 + rim diameter (for inch sizes, the first number is the diameter). Revolutions per mile = 63,360 ÷ (π × diameter). Actual speed = indicated speed × 31.00 ÷ 28.88. Real tires vary by brand and model, so treat these as close estimates and use manufacturer specs for exact measurements.
Frequently asked questions
How much bigger is 31x10.50R15 than 235/75R15?
31x10.50R15 is 2.12 inches (7.3%) taller in overall diameter (28.88 in vs 31.00 in) and +1.25 inches in section width.
Will my speedometer be off with 31x10.50R15 instead of 235/75R15?
Yes, by about 7.3%. If the speedometer is set up for 235/75R15, it will show 60 mph when you are really going about 64.4 mph.
Is 31x10.50R15 within 3% of 235/75R15?
No. The diameter difference is +7.3%. Check clearance and consider recalibrating the speedometer.