ElpiAuto30X9.50R15 vs 235/75R15
235/75R15 is 3.7% shorter (29 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 — 235/75R15
- 235section width9.3″ across the tread, sidewall to sidewall
- 75aspect ratiosidewall is 75% of the width — 6.9″ tall
- Rconstructionradial — standard on every modern road car
- 15wheel diametermounts on a 15-inch wheel
In flotation sizing: ≈ 28.9x9.3R15
30X9.50R15 vs 235/75R15, measurement by measurement
Every dimension in inches, with the difference and which size is bigger
| Measurement | 30X9.50R15 | 235/75R15 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 30.00″ | 28.88″ | −1.12″ | −3.7% |
| Section width | 9.50″ | 9.25″ | −0.25″ | −2.6% |
| Sidewall height | 7.50″ | 6.94″ | −0.56″ | −7.5% |
| Circumference | 94.25″ | 90.72″ | −3.53″ | −3.7% |
| Revs per mile | 672.3 | 698.4 | +26.1 | +3.9% |
| Revs per km | 417.7 | 434.0 | +16.2 | +3.9% |
| Approved rim width estimate | 6.5–9″ | 6.5–9″ | — | — |
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 235/75R15. 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 57.8 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 over-counts by roughly 389 miles.
| Speedometer reads | True speed on 235/75R15 | Error |
|---|---|---|
| 20 mph | 19.3 mph | −0.7 |
| 30 mph | 28.9 mph | −1.1 |
| 40 mph | 38.5 mph | −1.5 |
| 50 mph | 48.1 mph | −1.9 |
| 60 mph | 57.8 mph | −2.2 |
| 70 mph | 67.4 mph | −2.6 |
| 80 mph | 77.0 mph | −3.0 |
With 30X9.50R15 as the original size. A shorter tire makes the dial read high — the gap grows with speed.
Reading this comparison
The overall diameter is what matters here: 30X9.50R15 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.