Tire Size Calculator
Enter your current and new tire sizes to compare diameter, sidewall height, circumference and revolutions per mile, see how far off your speedometer will read, and view both tires drawn to scale.
| Dimension | Original | New | Difference |
|---|
Side-by-side drawing (to scale)
Speedometer error
| Speedometer shows | Actual speed | Difference |
|---|
Assumes the speedometer was accurate with the original tires.
How to use the Tire Size Calculator
- Type your original tire size as printed on the sidewall, for example 225/45R17, or fill in width, aspect ratio and rim separately.
- Type the new size you are considering in the second box.
- Press Calculate & compare (results also update as you type) and read the diameter difference at the top.
- Check the speedometer table to see your real speed for each speedometer reading. Switch between mph and km/h.
- Look at the drawing to see the size difference to scale, and heed the warning if the diameters differ by more than 3%.
Tire size formulas
A metric tire size such as 225/45R17 contains everything needed to work out the tire's dimensions:
- Sidewall height = width × aspect ratio ÷ 100 = 225 × 45 ÷ 100 = 101.25 mm
- Overall diameter = rim × 25.4 + 2 × sidewall = 431.8 + 202.5 = 634.3 mm (24.97 in)
- Circumference = π × diameter = 3.1416 × 634.3 = 1,992.7 mm
- Revolutions per mile = 1,609,344 ÷ circumference = 807.6 (per km: 1,000,000 ÷ 1,992.7 = 501.8)
Flotation sizes used on trucks and off-road tires, such as 31x10.50R15, state the overall diameter (31 in), width (10.50 in) and rim (15 in) directly.
Worked comparison: 225/45R17 vs 235/40R18
The 235/40R18 has a 94 mm sidewall and a 645.2 mm (25.40 in) diameter. Compared with 634.3 mm, that is 10.9 mm taller, a difference of +1.72%. When the speedometer shows 60 mph you are really doing 60 × 645.2 ÷ 634.3 = 61.0 mph. Because the difference is under 3%, this is a common plus-one upgrade.
Speedometer error at a glance
| Diameter change | Speedometer shows 30 mph | Shows 60 mph | Shows 100 km/h |
|---|---|---|---|
| −3% | 29.1 mph | 58.2 mph | 97 km/h |
| −1% | 29.7 mph | 59.4 mph | 99 km/h |
| +1% | 30.3 mph | 60.6 mph | 101 km/h |
| +3% | 30.9 mph | 61.8 mph | 103 km/h |
| +5% | 31.5 mph | 63.0 mph | 105 km/h |
Tips before changing tire size
- Stay within about ±3% of the original diameter to keep speedometer, ABS and traction control readings sensible.
- Wider tires need a rim width the tire maker approves, and enough room at full steering lock and suspension compression.
- Match or exceed the original load index and speed rating shown on your door placard.
- When going up a rim size ("plus sizing"), lower the aspect ratio so the overall diameter stays close to the original.
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the section width in millimeters, 45 is the aspect ratio (the sidewall height is 45% of the width), R means radial construction and 17 is the wheel rim diameter in inches.
How is tire diameter calculated?
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter × 25.4 + 2 × sidewall height. For 225/45R17 the sidewall is 225 × 0.45 = 101.25 mm and the diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 inches.
How much can tire diameter change safely?
Most tire fitters and manufacturers recommend keeping the new overall diameter within about 3% of the original. Larger changes can make the speedometer, ABS and stability control less accurate and may cause rubbing.
Will bigger tires make my speedometer read slower?
Yes. A larger tire covers more ground per revolution, so you travel faster than the speedometer shows. Actual speed = indicated speed × new diameter ÷ old diameter. With a 3% larger tire, 60 mph on the speedometer is really about 61.8 mph.
What does revolutions per mile mean?
It is how many times the tire turns to cover one mile: 1 mile (1,609,344 mm) divided by the circumference. Smaller tires turn more times per mile, which also affects odometer readings.
Is this calculation exact for my tire?
It uses the nominal dimensions from the size code. Real tires vary a little by brand, tread depth, inflation and load, so actual measurements can differ by a few millimeters.