Tire Size Calculator

A size like 225/45R17 has three parts. The 225 is the tread width in millimetres. The 45 is the aspect ratio, meaning the sidewall height is 45% of the width. The R means radial construction, and the 17 is the wheel diameter in inches. From those you can work out the overall tire diameter, which is what changes your speedometer and gearing when you switch sizes.

Your speedometer is calibrated for the original tire's diameter, so it counts wheel rotations and assumes each one covers a set distance. A larger diameter tire travels farther per rotation, so you're actually going faster than the dial shows; a smaller one, slower. This calculator shows the exact error at any speed, so you can see whether a size change nudges you into an unintended speed.

A common rule of thumb is to keep the overall diameter within about 3% of the original, which usually keeps the speedometer accurate enough and avoids rubbing or clearance problems. Bigger changes can affect braking, the anti-lock and stability systems, gearing, and fuel economy, and may need a speedometer recalibration. The calculator flags the percentage change so you can judge it.

Revolutions per mile is how many times a tire turns to cover one mile. It comes straight from the circumference: a bigger tire has a longer circumference, so it turns fewer times per mile. It matters because your speedometer, odometer, and the car's computer all rely on a fixed revs-per-mile value, so changing it changes what they report.

Plus-sizing means fitting a larger wheel with a lower-profile tire so the overall diameter stays close to stock, keeping the speedometer honest while improving looks and handling. The trick is matching the diameter: as the rim goes up an inch, the aspect ratio comes down to compensate. Enter both sizes here and aim for a diameter change near zero, then confirm the width and offset clear your fenders and suspension.

It can. A larger diameter tire effectively raises your gearing, which can slightly lower engine RPM at cruising speed and nudge highway economy up, but it makes acceleration feel a little slower and can hurt around town. A smaller diameter does the opposite. The effects are usually modest within a few percent of diameter, which is another reason to keep the change small.