Tire Size Calculator

Inputs

Old section width (W1)

First number in the old tire size, such as 225 in 225/45R17.

Old aspect ratio (A1)

Sidewall height as a percentage of section width.

Old rim diameter (R1)

Wheel rim diameter in inches.

New section width (W2)

First number in the new tire size.

New aspect ratio (A2)

New sidewall height as a percentage of section width.

New rim diameter (R2)

New wheel rim diameter in inches.

Indicated speed (v_i)

Speed shown on the speedometer with the original tire size.

km/h

Default result

New tire diameter

653.2mm

Pass

Also computed

Diameter change2.98%

Within the common +/-3% speedometer screening range.

True speed103km/h

At the entered indicated speed.

Speedometer change2.98%

Old tire diameter634.3mm

New circumference2,052mm

Revolutions per kilometre487.3rev/km

Method notes 2 notes

Tire notation is handled conventionally: width in mm, aspect ratio in percent, and rim diameter in inches.
Actual rolling radius varies with tire model, load, inflation pressure, tread depth and speed. Use manufacturer data for final gearing or calibration work.