Tire Size Calculator

225 is the section width in millimetres, 45 is the aspect ratio (sidewall height as a percent of the width), R marks radial construction, and 17 is the rim diameter in inches. This tool takes those three numbers, one per box, and leaves the R implied.

First the sidewall height in millimetres is width times aspect ratio divided by 100. That height sits above and below the rim, so the diameter is the rim plus two sidewalls converted to inches: rim + 2 x sidewall / 25.4. For 225/45R17 that is 17 + 2 x 101.25 / 25.4, which comes to 24.97 inches.

It is how many full rotations the tire makes to travel one mile. The tool divides 63,360 inches in a mile by the tire circumference in inches, where circumference is pi times the overall diameter. A 225/45R17 turns about 807.6 times per mile, and a taller tire turns fewer times.

A taller tire covers more ground per rotation, so the wheel sensor counts fewer turns for the same distance and the speedometer reads slower than you are actually going, while the odometer undercounts miles. Fit a shorter tire and the opposite happens. The compare box reports this gap as a signed percentage.

The percentage is measured from your base size to the compare size. A positive number, like +8.83% for 235/55R17 against 225/45R17, means the compare tire is larger, so a speedometer calibrated for the base would read low. A negative number means the compare tire is smaller and the speedometer would read high.

A common guideline is to keep overall diameter within about 3 percent of the original so gearing, speedometer accuracy and traction systems stay close to factory. This tool shows you the exact diameter and percent difference, but you must still check width fit, load rating and clearance against your vehicle.

No. It reads standard P-metric sizes given as width in millimetres, aspect ratio and rim in inches. Flotation sizes like 33x12.50R15 use inches for diameter and width directly, so they do not fit these three boxes and would need a different formula.

Width, aspect ratio and rim must each be positive numbers or the tool shows a prompt instead of results, and the same rule applies to the three compare fields when the compare box is ticked. There is no upper cap, so you can model unusual sizes, but the geometry is only meaningful for realistic tire dimensions.

No. The calculation runs entirely in your browser using the same formula every time, so nothing about your vehicle or the sizes you try is uploaded or stored. You can use it offline once the page has loaded.