Tire Size Calculator & Comparison
Enter your current tire size and the new size you're considering (the numbers on the sidewall, like 225/45R17). See how the diameter changes and what your speedometer will really read.
How to read a tire size
In a size like 225/45R17: 225 is the tread 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 diameter in inches.
Tire size formulas
Sidewall (in) = width (mm) × aspect ratio ÷ 100 ÷ 25.4
Overall diameter (in) = wheel diameter + 2 × sidewall
Revolutions per mile = 63,360 ÷ (π × diameter)
Speedometer reading error = (new diameter − old diameter) ÷ old diameter
Example: 225/45R17 vs 235/40R18
The 225/45R17 has a 3.99 in sidewall and a 24.97 in overall diameter. The 235/40R18 has a 3.70 in sidewall and a 25.40 in diameter — about 1.7% larger. When your speedometer shows 60 mph you would actually be going about 61 mph.
The ±3% rule
Many tire shops recommend keeping the new overall diameter within about 3% of the original to avoid rubbing, speedometer error and effects on ABS, traction control and transmission shift points. Also match or exceed the original load index and speed rating, and check wheel width and offset.
Frequently asked questions
What does 225/45R17 mean?
225 mm tread width, a sidewall 45% as tall as the width, radial construction, for a 17-inch wheel.
How much bigger can I go on tire size?
A common guideline is to stay within about 3% of the original overall diameter. Bigger changes can cause rubbing and speedometer, ABS or transmission issues.
Will bigger tires change my speedometer?
Yes. Larger diameter tires make your speedometer read lower than your true speed; smaller tires make it read higher.
Can I put wider tires on the same wheel?
Often one size wider is fine, but every tire has an approved wheel-width range. Check the tire maker's specifications.