235/75R15 vs 31x10.50R15
Switching from 235/75R15 to 31x10.50R15 changes the overall diameter by +53.9 mm (+7.3%) — outside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 235/75R15 | 31x10.50R15 | Difference |
|---|---|---|---|
| Overall diameter | 733.5 mm | 787.4 mm | +53.9 mm (+7.3%) |
| Overall diameter | 28.88 in | 31.00 in | +2.12 in |
| Tire width | 235 mm (9.25 in) | 267 mm (10.50 in) | +32 mm |
| Sidewall height | 176.3 mm (6.94 in) | 203.2 mm (8.00 in) | +26.9 mm |
| Wheel (rim) diameter | 15.0 in | 15.0 in | 0.0 in |
| Circumference | 2304 mm | 2474 mm | +169 mm |
| Revolutions per mile | 2194 | 2044 | -150 |
| Revolutions per km | 434 | 404 | -30 |
Speedometer error
Calibrated for 235/75R15, the speedometer under-reads or over-reads once 31x10.50R15 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 31x10.50R15 | Error |
|---|---|---|
| 20 mph | 21.5 mph | +1.5 mph |
| 40 mph | 42.9 mph | +2.9 mph |
| 60 mph | 64.4 mph | +4.4 mph |
| 80 mph | 85.9 mph | +5.9 mph |
| 100 mph | 107.3 mph | +7.3 mph |
How these two sizes are worked out
235/75R15 breaks down as a 235 mm tread width with a sidewall of 75% of that width (176.3 mm) on a 15.0-inch wheel, giving an overall diameter of 733.5 mm (28.88 in). 31x10.50R15 works out to 787.4 mm (31.00 in) the same way. The standard formulas are the same ones used on the tire size calculator page: diameter = rim × 25.4 + 2 × sidewall, circumference = π × diameter, revolutions per mile = 63,360 ÷ diameter in inches.
Nominal sizes vary slightly between brands and models, and tread wear shrinks a tire over its life. These figures follow the size codes exactly; your vehicle manual or a professional tire shop is the authority for what fits your vehicle.
More comparisons
Frequently asked questions
Can I replace 235/75R15 tires with 31x10.50R15?
The diameter change is +7.3%, which is outside the commonly used ±3% guideline. Diameter is only one part of fitment — wheel width range, load index, speed rating and physical clearance all have to check out too, so confirm with your vehicle manual or a professional tire shop before buying.
Will 31x10.50R15 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 64.4 mph on 31x10.50R15. The full table above covers 20 to 100 mph.
Which tire is taller, 235/75R15 or 31x10.50R15?
31x10.50R15 is taller — 787.4 mm against 733.5 mm overall diameter, a difference of 53.9 mm.