🛞 Tire Size Calculator
Enter a tire size code (e.g. 225/45R17) to calculate sidewall height, overall diameter and circumference, and optionally compare against a second size.
Reading the code and getting a diameter
In 225/45R17 the 225 is section width in millimetres, 45 is the aspect ratio as a percentage of that width, and 17 is the rim in inches:
sidewall mm = width x aspect / 100
diameter mm = rim inches x 25.4 + 2 x sidewall
circumference = diameter x piSo 225/45R17 has a 101.25 mm sidewall and a 634.3 mm overall diameter, which is 24.97 inches and 78.45 inches around. The sidewall is counted twice because there is one above the rim and one below it.
The comparison and speedometer error
Fill the second box and the two diameters are divided: 235/40R18 comes out at 25.40 inches, +1.72% on the original. A larger tire covers more ground per revolution than the car expects, so the speedometer under-reads by that percentage - an indicated 60 mph is really about 61. A smaller tire does the reverse and flatters your speed.
- Stay inside about 3%. That is the usual tolerance for keeping the speedometer, odometer and ABS calibration honest.
- The odometer drifts too. A 1.72% error is roughly 170 extra miles unrecorded over 10,000, which quietly shifts service intervals and warranty mileage.
Frequently asked questions
What does 225/45R17 mean?
A tread 225 mm wide, a sidewall 45% of that width at 101.25 mm, radial construction, on a 17-inch rim. Those three numbers are all the geometry here needs.
Will bigger tires make my speedometer wrong?
Yes, by the diameter difference. Going from 24.97 to 25.40 inches makes the speedometer read 1.72% low, so you are travelling slightly faster than it says.
Does the same size measure the same on every tire?
No. This is the nominal figure from the code. Real moulded sizes vary by a few millimetres between brands, and a loaded tire rolls at less than its free diameter, so treat the number as a comparison rather than a measurement.