Enter the three numbers from your tire's sidewall (like 225/45R17) and we will translate them into real measurements: overall diameter, sidewall height, and circumference. Switch to compare mode to check a second size against your original and see the speedometer error a swap would cause.
A tire size like 225/45R17 is three measurements in two different unit systems wearing a trench coat. The width is metric, the wheel is imperial, and the middle number is not a measurement at all but a percentage. Once you unpack it, every useful fact about the tire's geometry falls out: how tall it stands, how far it rolls per revolution, and what happens to your speedometer if you change it. This page does the unpacking.
Nothing you type here is stored or transmitted. All calculations run in your browser.
How tire sizing works
The section width (225) is the tire's widest point in millimeters, sidewall to sidewall. The aspect ratio (45) says the sidewall's height is 45 percent of that width, so a wider tire with the same aspect number has a taller sidewall too, which surprises people. The R is radial construction (effectively every passenger tire since the 1970s), and 17 is the diameter in inches of the wheel it mounts on. The tire's overall diameter is the wheel plus a sidewall above it and a sidewall below it.
The formula
sidewall (in) = width × aspect ÷ 100 ÷ 25.4
diameter (in) = wheel + 2 × sidewall
circumference (in) = π × diameter
revolutions per mile = 63360 ÷ circumference
Width is in millimeters, aspect is the percentage from the sidewall, wheel is in inches, and 25.4 converts millimeters to inches. There are 63,360 inches in a mile. When comparing two sizes, the diameter difference in percent is what matters, and your true speed equals indicated speed times the new diameter divided by the old.
Worked example
Take 225/45R17. Sidewall: 225 × 45 ÷ 100 = 101.25 mm, which is 101.25 ÷ 25.4 = 3.99 inches. Diameter: 17 + 2 × 3.99 = 25.0 inches (63.4 cm). Circumference: π × 24.97 = 78.5 inches, so the tire turns 63360 ÷ 78.45 = 808 times per mile. Now compare 245/40R18: sidewall 98 mm (3.86 in), diameter 25.7 inches. That is 2.98 percent taller, inside the 3 percent rule, and an indicated 60 mph becomes a true 61.8 mph.
The 3 percent rule
Tire shops and vehicle manufacturers converge on the same guidance: keep any replacement tire's overall diameter within about 3 percent of the original. It is not arbitrary. Your speedometer, odometer, ABS, traction control, and (on many cars) transmission shift points all assume the factory circumference. Inside 3 percent, the errors stay small and the electronics cope. Beyond it, your speedometer lies to you in whichever direction you changed, your odometer quietly miscounts every mile, and gearing effectively changes: taller tires raise your effective final drive, which dulls acceleration, while shorter ones do the reverse and raise engine revs at highway speed. If a swap you want shows more than 3 percent in the comparison above, that is the moment to talk to an alignment shop rather than a checkout page.
Frequently asked questions
What do the numbers on a tire size mean?
Take 225/45R17. The first number, 225, is the tire's section width in millimeters. The second, 45, is the aspect ratio: the sidewall's height as a percentage of that width, so here the sidewall is 45 percent of 225 mm. The R means radial construction, and 17 is the wheel diameter in inches that the tire fits. Letters after the size (like 94V) are the load index and speed rating.
Can I put a different tire size on my car?
Within limits. The widely used rule of thumb is to keep the new tire's overall diameter within 3 percent of the original, which keeps your speedometer, odometer, ABS, and stability control reading close to the truth. You also need the width to clear the fenders and suspension. The calculator's compare mode shows the diameter difference directly, and anything beyond 3 percent deserves a conversation with a tire shop.
How does tire size affect my speedometer?
Your speedometer counts wheel revolutions and assumes the original tire's circumference. A larger diameter tire covers more ground per revolution, so the car is moving faster than the gauge says; a smaller tire does the opposite. The error scales exactly with the diameter ratio: a 3 percent larger tire means an indicated 60 mph is really about 61.8 mph.
What is plus sizing?
Fitting a larger wheel with a lower profile tire while keeping the overall diameter nearly unchanged. Going from 225/45R17 to 245/40R18 is a classic plus one: the wheel grows an inch, the sidewall shrinks to compensate, and the total diameter changes by only about 3 percent. Handling usually sharpens; ride comfort and pothole resistance usually suffer.
Does a wider tire mean a taller tire?
Usually yes, and this catches people out. Because the aspect ratio is a percentage of the width, a 245/45R17 has a taller sidewall than a 225/45R17 even though both say 45. The sidewall is 45 percent of a bigger number. That is why changing width alone still changes your overall diameter and speedometer accuracy.
What about tire sizes like 33x12.50R15?
That is flotation sizing, used mostly for off-road truck tires, and it is refreshingly literal: 33 inches overall diameter, 12.5 inches wide, 15 inch wheel. This calculator works with the metric format found on nearly all passenger cars. To compare a flotation size against a metric one, the flotation diameter is already the number you need.