Tire size calculator
Enter your current tire and a replacement to see the difference in diameter, revolutions per mile, speedometer error and true speed. Example: going from 265/70R17 to 285/70R17 changes diameter by +3.49%, so an indicated 60 mph is 62.1 mph.
Is this a safe swap?
Diameter difference +3.49%: outside the ±3% guideline
By overall diameter alone, 285/70R17 falls outside the ±3% range commonly used as a guideline when replacing 265/70R17; speedometer and ride-height changes are larger. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.
| Measurement | 265/70R17 | 285/70R17 | Difference | % diff |
|---|---|---|---|---|
| Overall diameter | 31.61 in (802.8 mm) | 32.71 in (830.8 mm) | +1.10 in (+28.0 mm) | +3.49% |
| Sidewall height | 7.30 in (185.5 mm) | 7.85 in (199.5 mm) | +0.55 in (+14.0 mm) | +7.55% |
| Section width | 10.43 in (265.0 mm) | 11.22 in (285.0 mm) | +0.79 in (+20.0 mm) | +7.55% |
| Circumference | 99.29 in (2,522.1 mm) | 102.76 in (2,610.0 mm) | +3.46 in (+88.0 mm) | +3.49% |
| Revolutions per mile | 638 | 617 | −22 | −3.37% |
| Indicated | Actual on 265/70R17 | Actual on 285/70R17 | Difference |
|---|---|---|---|
| 30 mph | 30.0 mph | 31.0 mph | +1.0 mph |
| 60 mph | 60.0 mph | 62.1 mph | +2.1 mph |
| 75 mph | 75.0 mph | 77.6 mph | +2.6 mph |
Ride-height change: 285/70R17 raises the vehicle by 0.55 in (14.0 mm) at the axle (half the +28.0 mm diameter difference).
Tire sizes by rim diameter
- 13-inch (9)
- 14-inch (21)
- 15-inch (40)
- 16-inch (45)
- 17-inch (64)
- 18-inch (64)
- 19-inch (46)
- 20-inch (61)
- 21-inch (26)
- 22-inch (29)
- 24-inch (7)
How the calculator works
Every number is calculated from the size itself using standard tire geometry. Nothing is looked up from a manufacturer catalog, so real tires can measure slightly differently.
- Sidewall height = width × aspect ratio ÷ 100
- Overall diameter (mm) = rim diameter × 25.4 + 2 × sidewall height
- Circumference = π × overall diameter
- Revolutions per mile = 63,360 ÷ circumference in inches
- Speedometer error % = (new diameter ÷ old diameter − 1) × 100
- True speed = indicated speed × new diameter ÷ old diameter
- Ride-height change = (new diameter − old diameter) ÷ 2
Flotation sizes such as 33x12.50R17 are read as overall diameter × section width (inches) and rim diameter, then converted to metric equivalents. See the methodology for details.
Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.