Tire Size Comparison Calculator

Compare metric tire sizes, nominal geometry, ride-height change, speedometer effect and RPM ratios.

Tire size change

Current tire diameter: in
New tire diameter: in
Diameter change, new minus current: in
Diameter change: %
Approximate ride-height change from tire radius: in
Actual speed at custom indicated speed: mph
Approximate RPM change at same actual road speed: %

Relative tire comparison

Both tires use the same relative scale, bottom-aligned. This is not physical screen scale or a fitment check.

Size comparison

Nominal tire geometry; changes are new minus current
Measurement	Current	New	Change

Speedometer effect

Assuming calibration for current tire remains unchanged
Indicated (mph)	Actual (mph)
30
45
55
65
70

Speedometer difference relative to indicated: %

Engine RPM at the same actual road speed

How it works

The first number is nominal width in millimeters; the aspect ratio expresses sidewall height as a percentage of width; R is radial notation and the wheel diameter is in inches. We add two sidewalls to wheel diameter to estimate overall tire diameter.

Diameter determines nominal circumference and revolutions per mile. The radius change is half the diameter change, so it is not a suspension lift. Larger tires travel farther each revolution: actual road speed is higher than indicated if calibration stays unchanged. Smaller tires do the opposite.

At the same actual road speed and unchanged gearing, larger tires reduce engine RPM and smaller tires increase it.

Assumptions and useful information

Conventional metric tire notation only; flotation sizes such as 33×12.50R15 are not supported. Inputs describe nominal geometry, not measured rolling dimensions. Results use unrounded calculations and round for readability.

Speedometer effects assume original calibration for the entered current tire. RPM effects assume the same actual road speed, transmission gear, final-drive ratio and converter/clutch state. Real tire model, pressure, load, tread and calibration can affect results.

The 3% diameter reference is informational only. Acceptable changes depend on the vehicle, wheels, clearances, ratings and calibration; it does not establish safety or fitment.