How to read a tire size
A tire size such as 225/50R17 packs three measurements into one code. The first number, 225, is the tire's section width in millimeters. The second, 50, is the aspect ratio: the sidewall height as a percentage of the width. The letter R means radial construction, and the last number, 17, is the wheel (rim) diameter in inches.
Many tires also show a load index and speed rating after the size, like 94V. Those do not change the dimensions, so this calculator does not need them.
How to calculate overall tire diameter
- Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in mm.
- Double it, because there is a sidewall above and below the wheel.
- Convert the rim from inches to mm by multiplying by 25.4.
- Add the rim and the two sidewalls.
Revolutions per mile and per km
The circumference is the diameter times pi, and it is the distance the tire rolls in one turn. Dividing a mile (1,609.34 m) or a kilometer (1,000 m) by the circumference gives the revolutions. A 225/50R17 has a circumference of about 2.06 m, so it turns roughly 780 times per mile. These values are theoretical. A loaded tire sits slightly lower than its unloaded height, so the real number of revolutions is a little higher.
Speedometer error after changing tire size
Your speedometer counts wheel rotations, not actual distance. If the new tire has a larger diameter, each rotation covers more ground, so the real speed is higher than what the dial shows. If the new tire is smaller, the dial reads high.
Use this rule: true speed = indicated speed × new diameter ÷ old diameter. The compare mode does this for you at 30, 60 and 70 mph. Odometer mileage is off by the same percentage, which matters for service intervals and warranties.
How much size change is safe?
A common guideline is to keep the new overall diameter within about 3 percent of the original. Larger differences can affect the speedometer, ABS and traction control calibration, gearing, and clearance in the wheel wells. Your vehicle placard or owner's manual lists approved sizes, and a tire shop can confirm what fits.
When you go to a bigger rim (plus sizing), lower the aspect ratio so the overall diameter stays about the same. For example, 225/50R17 and 235/45R18 are within about 2 percent.
Tire size reference
| Tire size | Diameter (in) | Diameter (mm) |
|---|---|---|
| 195/65R15 | 25.0 | 634.5 |
| 205/55R16 | 24.9 | 631.9 |
| 225/50R17 | 25.9 | 656.8 |
| 245/40R18 | 25.7 | 653.2 |
| 265/70R17 | 31.6 | 802.8 |
Things that change the real result
- Wheel width: the same tire on a wider or narrower rim changes its shape slightly.
- Wear: a worn tire is about 0.3 to 0.5 inch smaller in diameter than a new one.
- Pressure and load: low pressure or a heavy load lowers the rolling height.
- Manufacturer variation: published diameters can differ by a few millimeters between brands.
Tire size and gearing
Changing tire diameter also changes the effective gearing of the vehicle. A taller tire turns fewer times per mile, so the engine runs at slightly lower rpm at the same road speed. That can help highway fuel economy a little but can reduce acceleration and make towing harder. A shorter tire does the opposite. For most small changes within 3 percent the effect is minor, but on trucks with big off-road tires a gear ratio change is often recommended.
The revolutions per mile figure is the number to compare. If it drops by 2 percent, the engine rpm drops by about 2 percent at the same real speed. Larger tires can also rub on the fender or suspension at full steering lock, so check clearance before you buy.