Compare Two Tire Sizes and Correct the Displayed Speed
Enter standard metric tire codes such as 225/65R17 and 245/60R18. The calculator parses section width, aspect ratio and wheel diameter, then reports sidewall height, rolling diameter, revolutions per mile, clearance change and actual road speed.
Δ SPEED
Original and replacement tire
Tire and speed difference
Tire size difference formula
Sidewall height equals section width in millimeters multiplied by aspect ratio and divided by 100. Convert the two sidewalls to inches and add the wheel diameter to obtain overall tire diameter. Circumference is π times diameter. The actual speed equals displayed speed multiplied by new circumference divided by original circumference.
Positive versus negative speed error
A larger replacement tire travels farther per revolution, so actual speed is higher than the speedometer display. A smaller tire travels less, so actual speed is lower. The same ratio corrects odometer and trip-distance readings.
What does R mean in a tire code?
R identifies radial construction. The geometry parser accepts common forms with or without the R character.