Tire Size Calculator

Tire size is determined from the tire code printed on the sidewall. The section width (in mm) is multiplied by the aspect ratio percentage to get the sidewall height. The overall diameter is calculated as: (sidewall height × 2) converted to inches plus the rim diameter. Circumference is simply the diameter multiplied by π (3.14159).

A typical tire code like 225/65R17 breaks down as follows: 225 is the section width in millimeters, 65 is the aspect ratio (sidewall height as a percentage of section width), R means radial construction, and 17 is the rim diameter in inches. Some codes start with a letter like P (passenger) or LT (light truck) before the numbers.

Sidewall height is the distance from the rim edge to the outer tread of the tire. It's calculated by multiplying the section width by the aspect ratio: for example, a 225/65R17 tire has a sidewall of 225 × 0.65 = 146.25 mm (about 5.76 inches). Two sidewalls make up the difference between rim diameter and overall tire diameter.

When switching tire sizes, keep the overall diameter as close to stock as possible (within 3%) to avoid affecting speedometer accuracy, ground clearance, and odometer readings. Also ensure the new tire fits within your wheel well without rubbing. Wider tires may require wider rims, and very large diameter changes can affect braking and handling.

Your speedometer is calibrated for your stock tire's revolutions per mile. A larger tire travels farther per revolution, meaning the speedometer will read lower than your actual speed. A smaller tire does the opposite. The percentage difference in circumference between your old and new tires equals the speedometer error percentage.

Revolutions per mile (or per km) tells you how many times your tire rotates for each mile of travel. It's calculated as 63,360 (inches per mile) divided by tire circumference in inches. This metric is critical for speedometer calibration, gear ratio selection, and understanding fuel economy differences between tire sizes.

Generally, a tire's section width should be within a range of roughly 10–20 mm of the rim width in inches (converted). Most rim manufacturers publish approved tire width ranges for each rim width. Going too wide or too narrow can cause handling problems, uneven wear, and even tire bead failure. Always check your vehicle's manual or a fitment guide.

Plus-sizing means increasing rim diameter while decreasing tire aspect ratio to maintain similar overall diameter. Benefits include improved cornering response and aesthetics, but drawbacks include a harsher ride (less sidewall to absorb bumps), potential for increased road noise, and higher cost. Whether it's 'worth it' depends on your driving priorities.