Tire Size Calculator
Decode any tire size — or compare two sizes for speedometer correction and diameter difference.
Enter your tire size — found on the tire sidewall
How to Read a Tire Size
Tire sizes are printed on the sidewall in the format Width/Ratio R Wheel — for example 225/65R17:
| Part | Value | Meaning |
|---|---|---|
| 225 | 225 mm | Section width — widest point of the tire |
| 65 | 65% | Aspect ratio — sidewall height is 65% of width (146.25 mm) |
| R | Radial | Construction type (radial is standard) |
| 17 | 17 inches | Wheel (rim) diameter |
Common Tire Sizes — Quick Reference
| Tire Size | Diameter | Sidewall | Circumference | Revs/Mile |
|---|---|---|---|---|
| 185/65R15 | 23.47" | 4.73" | 73.73" | 860 |
| 195/65R15 | 24.99" | 4.97" | 78.51" | 807 |
| 205/55R16 | 24.88" | 4.44" | 78.16" | 811 |
| 215/60R16 | 25.59" | 5.08" | 80.38" | 788 |
| 225/65R17 | 28.52" | 5.76" | 89.60" | 707 |
| 235/65R17 | 28.98" | 6.01" | 91.03" | 696 |
| 245/70R17 | 30.47" | 6.75" | 95.71" | 662 |
| 265/70R17 | 31.61" | 7.30" | 99.29" | 638 |
| 275/55R20 | 31.91" | 5.95" | 100.23" | 632 |
| 285/70R17 | 32.71" | 7.78" | 102.75" | 616 |
Frequently Asked Questions
How do I read a tire size like 225/65R17?
225 is the tire width in mm. 65 is the aspect ratio — the sidewall height is 65% of 225mm = 146.25mm. R means radial construction. 17 is the wheel (rim) diameter in inches.
How is tire diameter calculated?
Tire diameter = (Sidewall height × 2) + (Wheel diameter in mm). Sidewall height = Width × Aspect ratio / 100. For 225/65R17: sidewall = 146.25mm, diameter = 146.25 × 2 + 17 × 25.4 = 724.3mm = 28.52".
How much tire size difference is acceptable?
Most manufacturers recommend keeping overall diameter within 3% of the original. Beyond that, the difference can affect speedometer accuracy, ABS/traction control calibration, and tire clearance inside the wheel well.
How does changing tire size affect the speedometer?
A bigger tire covers more ground per revolution, so the speedometer reads lower than your actual speed. A smaller tire causes the speedometer to read high. Use Compare mode above to calculate the exact error for any two sizes.
Can I replace 225/65R17 with 235/65R17?
Yes — the overall diameter increases from 28.52" to 28.98" (+1.6%). The speedometer will read about 1.6% low at highway speeds. This is within most acceptable ranges but verify clearance and check your vehicle's manual.
What is plus sizing?
Plus sizing means fitting a larger wheel with a lower-profile tire to maintain the same overall diameter. For example, going from 225/65R17 to 235/55R18 keeps a similar diameter while fitting an 18" wheel — improving handling with more rim and less sidewall flex.
What is aspect ratio?
The aspect ratio is the sidewall height expressed as a percentage of the tire width. A lower aspect ratio (e.g., 45) means a shorter, stiffer sidewall — common on sports cars. A higher aspect ratio (e.g., 70) means a taller sidewall — common on trucks and SUVs for more cushioning.
How many revolutions per mile does a tire make?
Revolutions per mile = 63,360 ÷ circumference (in inches). A 225/65R17 has a 89.6" circumference, giving it approximately 707 revolutions per mile. Larger tires make fewer revolutions per mile.