The Tire Size Calculator finds the diameter, width, sidewall height, circumference and revolutions per mile of a tire from its size code, and compares two tire sizes.
265/70R17: 31.61 inches tall
| Diameter | 31.61 in (803 mm) |
| Width | 10.43 in (265 mm) |
| Sidewall height | 7.30 in |
| Circumference | 99.29 in |
| Revolutions per mile | 638.1 |
Tire 2 is 0.82% smaller
| Diameter | 31.61 in / 31.35 in |
| Circumference | 99.29 in / 98.48 in |
| Revolutions per mile | 638.1 / 643.4 |
| Difference | -0.82% |
| Real speed when speedometer shows 60 mph | 59.5 mph |
A size such as 265/70R17 means a section width of 265 mm, a sidewall height that is 70% of the width, radial construction (R) and a 17-inch wheel. The sidewall height is 265 × 0.70 = 185.5 mm (7.30 in), so the overall diameter is 17 + 2 × 7.30 = 31.6 in.
Changing the overall diameter changes the speedometer reading, gearing, ground clearance and how the tire fits in the wheel well. Keeping a new size within about 3% of the original diameter is a common rule.
Worked example: switching from 265/70R17 (31.6 in) to 265/40R23 (31.3 in) is about 1% smaller; at an indicated 60 mph you would really be going about 59.4 mph.
Fitting a larger wheel with a lower-profile tire keeps the diameter similar while improving steering response, at the cost of a firmer ride and more risk of wheel damage. The "new wheel size" option finds the aspect ratio that keeps the diameter the same.
What do the load and speed codes mean? A number for maximum load and a letter for maximum speed, e.g. 112T.
Can I mix tire sizes? Not on the same axle; all-wheel-drive cars often need all four the same.
What is P or LT before the size? Passenger or light truck tires.