Tire size calculator
Type the size on your sidewall and, if you like, the size you are considering. The calculator parses both, works out every dimension from geometry, and draws them to scale so you can see where the extra sidewall or width goes. Nothing here is vehicle-specific: it is arithmetic on the size code.
| Dimension | Current | New | Difference | Formula |
|---|---|---|---|---|
| Section width | 225.0 mm | 235.0 mm | +10.0 mm | first number of the size (mm) |
| Sidewall height | 146.3 mm | 152.8 mm | +6.5 mm | width × aspect ÷ 100 |
| Rim diameter | 431.8 mm | 431.8 mm | 0.0 mm | rim inches × 25.4 |
| Overall diameter | 724.3 mm | 737.3 mm | +13.0 mm (+1.79%) | rim + 2 × sidewall |
| Circumference | 2275.5 mm | 2316.3 mm | +40.8 mm | π × overall diameter |
| Revolutions per mile | 707.3 | 694.8 | -12.5 | 1,609,344 mm ÷ circumference |
| Ride height change | — | — | +6.5 mm | diameter difference ÷ 2 |
Speedometer error with the new size
The speedometer counts wheel revolutions and assumes the original circumference. With a larger tire each revolution covers more ground, so the car goes faster than the needle says. Actual speed = indicated × (new diameter ÷ current diameter) = indicated × 1.0179.
| Speedometer reads | Actual speed | Error |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 mph |
| 45 mph | 45.8 mph | +0.8 mph |
| 60 mph | 61.1 mph | +1.1 mph |
| 75 mph | 76.3 mph | +1.3 mph |
What this calculator cannot tell you. It does not know your vehicle, so it cannot say whether the new size clears the fender, strut or inner liner, whether the load index is enough for your axle weight, or whether your manufacturer approves the change. Those answers come from the Tire and Loading Information label on the driver's door edge, the owner's manual and a fitter. A common rule of thumb in the trade is to keep overall diameter within about ±3% of the original; that is a convention, not a law, and ABS, stability control and gearing all react to diameter, so treat anything outside it as a question for the fitter.
How the tire size calculator works
A metric size such as 225/65R17 carries three numbers. The first is the section width in millimetres. The second is the aspect ratio: sidewall height as a percentage of that width. The third is the rim diameter in inches. From those three, every other dimension follows:
sidewall = width × aspect ÷ 100 diameter = rim × 25.4 + 2 × sidewall circumference = π × diameter revs/mile = 1,609,344 ÷ circumference
A flotation size such as 33x12.50R17 gives the overall diameter and width in inches directly; the calculator converts them at exactly 25.4 mm per inch and derives the sidewall from the rim. These are nominal, unloaded figures from the size code. A mounted tire's measured diameter differs slightly by brand, wheel width and inflation, which is why the output shows one decimal and no more.
The drawing is a section through the axle. The rim band is drawn at the rim diameter, and the two tire profiles sit on it with their true width and sidewall height, all at the same scale, so a 10 mm wider tire looks 10 mm wider. For what each letter and number on the sidewall means, including the load index and speed symbol tables, read what a tire size means. If you are changing wheels as well as tires, the wheel offset calculator shows where the new wheel sits relative to the hub.