Tire size calculator
Enter your tire size and a new one, compare them side by side.
What does 225/65R17 mean?
225 is the tread width in millimeters, 65 the sidewall height as a percent of that width (146 mm), R means radial, and 17 the wheel diameter in inches. Put together, the tire is about 28.5 inches tall.
How do I work out a tire's diameter?
Diameter = wheel + 2 × sidewall. For 225/65R17: sidewall = 225 × 0.65 = 146.25 mm = 5.76 in, so 17 + 2 × 5.76 = 28.5 in.
How do bigger tires change my speedometer?
The speedometer counts wheel turns, so a taller tire covers more road per turn than it expects. Going from 225/65R17 to 245/70R17 (30.5 in), an indicated 60 mph is really 64.2 mph.
What are revolutions per mile?
How many times the tire turns in a mile: 63,360 inches ÷ the circumference. A 28.5-inch tire turns about 707 times per mile.
What about sizes like 33x12.50R15?
Those "flotation" sizes give inches directly: 33 in tall, 12.5 in wide, for a 15-inch wheel. The calculator reads both styles.
Worked example: a larger wheel with a lower profile
Compare the stock 225/65R17 with a 255/55R19. The stock sidewall is 225 × 0.65 = 146.3 mm (5.76 in) and the overall diameter 28.5 in. The new tire has a sidewall of 255 × 0.55 = 140.3 mm (5.52 in), and with a 19-inch wheel the diameter is 19 + 2 × 5.52 = 30.0 in, which is 5.4% larger. The wheel is 2 inches bigger, but the lower sidewall means the overall diameter grows only 1.5 in. The tire turns 671 times per mile instead of 707, 36 fewer. A speedometer reading 65 mph then means 68.5 mph, and the odometer, which counts turns in the same way, shows 100 miles after 105.4 miles. A flotation size such as 33x12.50R15 is read directly: 33.0 in tall, 15 in rim, 611 turns per mile.
Which numbers change the speedometer, and which do not?
Only the overall diameter matters for the speedometer and odometer: the true speed is the indicated speed times the new diameter divided by the old one. Width on its own does not change it, and neither do the load index and speed rating at the end of the sidewall code, which describe capacity (see the tire load index calculator). A change in diameter also shifts the effective gearing: a taller tire turns fewer times per mile, so the engine runs at a lower speed at the same road speed. Whether a new size fits under the fenders and on the wheel is a separate question that depends on width, offset and clearance.
Common mistakes with tire sizes
Treating the aspect ratio as the sidewall height: 65 is a percentage, and the height is 225 × 0.65 = 146.3 mm only because it is a percent of the width. Comparing wheel diameters instead of overall tire diameters: a 19-inch wheel does not make a tire 2 inches larger when the sidewall shrinks. Mixing metric and inch sizing styles. Forgetting that the speedometer error grows with speed in proportion: at 5.4% larger, the error at 65 is 3.5 mph. And forgetting that the odometer error accumulates with mileage.
Sources and related calculators
Sources: tire size geometry (width in millimeters, aspect ratio in percent of width, rim diameter in inches; 1 inch = 25.4 mm exactly), and the proportionality of speedometer reading to wheel diameter. Related: tire load index calculator, wheel offset calculator, RPM to MPH calculator and speed calculator.