About

Altering the wheel or tire size of a vehicle changes the overall rolling circumference, decoupling the physical speed of the vehicle from the mechanical reading of the speedometer. Speedometers are calibrated to the rotation of the factory-specified tire. If a larger diameter tire is installed, the vehicle travels a greater distance per axle rotation than the sensor expects, resulting in a speedometer reading that is lower than actual speed. This calculator quantifies that variance. Precise calculation is necessary to avoid unintentional speeding violations and to maintain the integrity of Anti-lock Braking Systems (ABS) and traction control, which rely on expected rotational velocity deltas.

Formulas

The total diameter (D) of a tire is the sum of the wheel diameter and the two sidewall heights. The formula is:

D = (W × A1270) + d

Where W is Section Width (mm), A is Aspect Ratio (e.g., 45 for 45%), and d is the Rim Diameter (inches). The divisor 1270 converts the double sidewall height from millimeters to inches.

Reference Data

Tire Code	Width (mm)	Aspect Ratio	Rim (in)	Diameter (in)
195/65R15	195	65%	15	25.0
205/55R16	205	55%	16	24.9
225/45R17	225	45%	17	25.0
225/40R18	225	40%	18	25.1
235/35R19	235	35%	19	25.5
245/30R20	245	30%	20	25.8
285/75R16	285	75%	16	32.8
315/70R17	315	70%	17	34.4

Frequently Asked Questions

A diameter variance exceeding 3% can cause failure in the ABS (Anti-lock Braking System) and ESP (Electronic Stability Program). These computers interpret the altered wheel speed as wheel slip or lock-up, potentially engaging brakes unexpectedly or disabling safety features.

Yes. If the tire is larger than stock, the odometer will register fewer miles than actually traveled. If the tire is smaller, it will register more miles. This affects resale value and maintenance intervals.

Speedometers count axle revolutions. A larger tire covers more ground per revolution. Since the car doesn't know the tire grew, it assumes the standard distance per revolution, resulting in a displayed speed that is lower than the actual ground speed.

On modern electronic vehicles, recalibration often requires an ECU tune or a specific handheld programmer to update the "Rev Per Mile" parameter. Older mechanical speedometers require changing the drive gear in the transmission.