Tire Size Comparison Calculator

Reading a tire size: in 225/45R18, 225 is nominal section width in millimetres, 45 is sidewall height as a percentage of width, R means radial construction, and 18 is wheel diameter in inches.

Each tyre's overall diameter = wheel diameter + 2 × (section width × aspect ratio). The speed error assumes tyre B is fitted but the speedometer remains calibrated for tyre A - actual speed = indicated speed × (diameter B ÷ diameter A).

Worked example: the starting 225/45R18 tire is 659.7 mm in diameter. The proposed 245/40R18 is 653.2 mm, a 6.5 mm or 0.99% reduction, and its circumference is about 20.4 mm shorter. With the speedometer still calibrated for tire A, an indicated 60 mph corresponds to about 59.41 mph. These are geometric comparisons, not a fitment approval.

A diameter change within about 3% is a common screening reference, not a universal fitment rule. Load rating, wheel width and offset, brake clearance, suspension clearance and the vehicle manufacturer's approved sizes still need to be checked.

Tyre dimensions are nominal. Measure the fitted tyre when clearance is close because rim width, pressure, load and manufacturer construction change the real section width and diameter.