How it works
A tire size like 225/65R17 means a 225 mm section width, a sidewall height that is 65% of the width, and a 17-inch wheel. The overall diameter is the wheel diameter plus two sidewalls.
Your speedometer counts wheel revolutions. If the new tire is taller, each revolution covers more ground, so you travel faster than the speedometer shows. A shorter tire makes the speedometer read high.
Tire shops generally recommend staying within about 3% of the original diameter. Always check load rating, speed rating and clearance for your vehicle before changing sizes.
Formula
Sidewall = width × aspect ÷ 100; diameter = 2 × sidewall + rim × 25.4; actual speed = indicated × new diameter ÷ old diameter
Questions people ask
How much tire size difference is acceptable?
Most guidance says keep the overall diameter within 3% of the original. Larger changes can throw off the speedometer, odometer, ABS and transmission shift points.
Will bigger tires make my speedometer wrong?
Yes. A tire 3% taller makes you go about 61.8 mph when the speedometer reads 60. Many vehicles can be recalibrated by a dealer or with a tuner.
What does the R in 225/65R17 mean?
Radial construction, which almost all modern passenger tires use.