235/75R15 vs 30x9.50R15
30x9.50R15 is outside the ±3% rule as a replacement for 235/75R15: the diameter differs by +3.89%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Side by side
| 235/75R15 | 30x9.50R15 | Difference | |
|---|---|---|---|
| Overall diameter | 28.88" | 30.00" | +1.12" (+3.89%) |
| Section width | 235 mm | 241 mm | +6 mm |
| Sidewall | 6.94" | 7.50" | +0.56" |
| Rim | 15" | 15" | same |
| Circumference | 90.7" | 94.2" | +3.5" |
| Revs per mile | 698 | 672 | -26 |
| Ride height | — | — | +0.56" |
| Speedometer reads 60 mph | 60.0 | 62.3 | +2.3 mph |
Speedometer and odometer
With 30x9.50R15 on a car calibrated for 235/75R15, the speedometer reads 57.8 mph when you're really doing 60, and the odometer under-counts by 3.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.7 mph |
| 60 mph | 62.3 mph |
| 70 mph | 72.7 mph |
| 80 mph | 83.1 mph |
Other sizes close to 235/75R15
Questions
Can I replace 235/75R15 with 30x9.50R15?
30x9.50R15 is outside the ±3% rule as a replacement for 235/75R15: the diameter differs by +3.89%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 235/75R15 and 30x9.50R15?
30x9.50R15 is 1.12" taller and 6 mm wider; the sidewall is +0.56". At an indicated 60 mph you'd actually be doing 62.3 mph.
Will 30x9.50R15 fit on 235/75R15 wheels?
Both use a 15" rim, so the wheel diameter matches; check the wheel's width range for a 241 mm tire and the fender clearance.