ElpiAuto30X9.50R15 vs 31X10.50R15
31X10.50R15 is 3.3% taller (25 mm) than 30X9.50R15 — outside the ±3% swap band.
Two sizes
Also accepts 35X12.50R17 and 31x10.5-15.
Both sizes, drawn to scale
1 grid = 1 inWhat each number means
The two size codes broken down — which digit is which, and what it does on the car.
Size A — 30X9.50R15
- 30overall height30.0″ tall — stated directly, no math
- 9.5section width9.5″ across the tread
- R15wheel diametermounts on a 15-inch wheel
In metric sizing: ≈ 240/80R15
Size B — 31X10.50R15
- 31overall height31.0″ tall — stated directly, no math
- 10.5section width10.5″ across the tread
- R15wheel diametermounts on a 15-inch wheel
In metric sizing: ≈ 265/75R15
30X9.50R15 vs 31X10.50R15, measurement by measurement
Every dimension in inches, with the difference and which size is bigger
| Measurement | 30X9.50R15 | 31X10.50R15 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 30.00″ | 31.00″ | +1.00″ | +3.3% |
| Section width | 9.50″ | 10.50″ | +1.00″ | +10.5% |
| Sidewall height | 7.50″ | 8.00″ | +0.50″ | +6.7% |
| Circumference | 94.25″ | 97.39″ | +3.14″ | +3.3% |
| Revs per mile | 672.3 | 650.6 | −21.7 | −3.2% |
| Revs per km | 417.7 | 404.3 | −13.5 | −3.2% |
| Approved rim width estimate | 6.5–9″ | 7.5–10″ | — | — |
Overall diameter is the controlling number — clearance, gearing, speedometer accuracy and odometer reading all follow from it. Revs per mile/km is what your ABS, speedometer and odometer actually count.
What changes on the road
Direction and rough size of each effect going from 30X9.50R15 to 31X10.50R15. Geometry only — real tires vary by brand and compound.
Speedometer & odometer error
What changes on the dash when the overall diameter changes.
A speedometer doesn't measure distance. It counts how fast the wheels turn and multiplies by a fixed factory number for tire circumference. Change the tire and that factory number is now wrong.
Going to a bigger tire, each turn covers more ground, so at a true 62.0 mph the wheels spin at the rate the factory calls 60. The dial reads low — you are speeding without knowing it. A smaller tire does the opposite, and the odometer also over-counts miles you didn't drive.
Over 10,000 indicated miles, this change means the odometer under-counts by roughly 323 miles.
| Speedometer reads | True speed on 31X10.50R15 | Error |
|---|---|---|
| 20 mph | 20.7 mph | +0.7 |
| 30 mph | 31.0 mph | +1.0 |
| 40 mph | 41.3 mph | +1.3 |
| 50 mph | 51.7 mph | +1.7 |
| 60 mph | 62.0 mph | +2.0 |
| 70 mph | 72.3 mph | +2.3 |
| 80 mph | 82.7 mph | +2.7 |
With 30X9.50R15 as the original size. A taller tire makes the dial read low — the gap grows with speed.
Reading this comparison
The overall diameter is what matters here: 31X10.50R15 is the taller tire. It sets ride height, gearing and how far the tire travels per revolution — which is what the speedometer and odometer count. Width and sidewall change fit and feel (a wider tire needs a wider wheel and more clearance; a shorter sidewall sharpens steering) but not the gearing.
Figures are geometric estimates from the size codes; real moulded tires vary by brand by 1–3%. Confirm load rating, wheel width and physical clearance on your vehicle before fitting. To compare any other two sizes, use the tire size comparison tool; to decode one size and see what else fits its wheel, the tire size calculator.