Tire size comparison
235/75R15 vs 30x9.50R15
30x9.50R15 is +3.9% taller than 235/75R15: 30.0 in vs 28.9 in (+1.12 in), and 0.2 in wider. With 30x9.50R15 on a vehicle set up for 235/75R15, the speedometer shows 60 mph at an actual 62.3 mph. That is more than the ±3% difference many shops use as a rule of thumb, so check carefully before switching.
Set up for: going bigger (30x9.50R15 instead of 235/75R15) · Different situation?
Your current size
e.g. 265/70R17, 2657017 or 33x12.50R20
Where do I find my size?
It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. Digits are enough here: 2657017 or 33125020.
Size you're considering
235/75R15 vs 30x9.50R15: full comparison
| 235/75R15 | 30x9.50R15 | Difference | |
|---|---|---|---|
| Overall diameter | 28.88 in | 30.00 in | +1.12 in (+3.9%) |
| Width | 9.25 in | 9.50 in | +0.25 in (+2.7%) |
| Sidewall height | 6.94 in | 7.50 in | +0.56 in |
| Circumference | 90.7 in | 94.2 in | +3.5 in |
| Revs per mile (nominal) | 698 | 672 | −26 |
| Wheel diameter | 15 in | 15 in | ±0 in |
| Diameter in mm | 733 | 762 | +29 mm |
Speedometer difference
Your speedometer is calibrated for 235/75R15. Fitted with 30x9.50R15, the actual speed is the reading × 30.00 ÷ 28.88.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.8 mph | +0.8 mph |
| 30 mph | 31.2 mph | +1.2 mph |
| 45 mph | 46.7 mph | +1.7 mph |
| 55 mph | 57.1 mph | +2.1 mph |
| 60 mph | 62.3 mph | +2.3 mph |
| 70 mph | 72.7 mph | +2.7 mph |
| 80 mph | 83.1 mph | +3.1 mph |
Real tires: published diameters
235/75R15: 28.7–29.1 in published by 5 makers; 30x9.50R15: 29.5–30 in published by 4 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 235/75R15 · 30x9.50R15
Related comparisons
FAQ
Which is bigger, 235/75R15 or 30x9.50R15?
30x9.50R15 is +3.9% taller than 235/75R15: 30.0 in vs 28.9 in (+1.12 in).
How much will my speedometer be off with 30x9.50R15 instead of 235/75R15?
The speedometer is calibrated for 235/75R15. With 30x9.50R15 it shows 60 mph when you are doing 62.3 mph, and 70 mph at 72.7 mph (+3.9%). Your speedometer also has its own factory tolerance.
Can I replace 235/75R15 with 30x9.50R15?
The diameter difference is +3.9%. Whether it fits depends on things this calculation can't see: wheel width and offset, clearance at full lock and over bumps, load index and speed rating, and your vehicle maker's approval. Check your door-jamb placard and owner's manual and ask a tire professional.