Speedometer difference
With a speedometer calibrated for the 225/75R15, this is your real speed on 235/75R15 tires.
| Speedometer shows | Actual speed with 235/75R15 | Difference |
|---|---|---|
| 20 mph | 20.4 mph | +0.4 mph |
| 30 mph | 30.6 mph | +0.6 mph |
| 40 mph | 40.8 mph | +0.8 mph |
| 50 mph | 51.0 mph | +1.0 mph |
| 60 mph | 61.3 mph | +1.3 mph |
| 70 mph | 71.5 mph | +1.5 mph |
| 80 mph | 81.7 mph | +1.7 mph |
Fitment considerations
- Width: the 235/75R15 is 0.39 in wider in section width, so each sidewall sits roughly 0.20 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 235/75R15.
- Height: the extra 0.59 in is a modest change, but still check the gap at full compression and full lock, especially with aftermarket wheels or added weight.
- Speedometer: a 2.1% change is small, but it adds up on the odometer: 1,000 miles shown would be about 1,021 real miles.
- 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 × 28.88 ÷ 28.29. 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 235/75R15 than 225/75R15?
235/75R15 is 0.59 inches (2.1%) taller in overall diameter (28.29 in vs 28.88 in) and +0.39 inches in section width.
Will my speedometer be off with 235/75R15 instead of 225/75R15?
Yes, by about 2.1%. If the speedometer is set up for 225/75R15, it will show 60 mph when you are really going about 61.3 mph.
Is 235/75R15 within 3% of 225/75R15?
Yes. The diameter difference is +2.1%.