Compare two tire sizes before you buy
Enter your current size and the size you are considering. You get the diameter of each, the difference, and what your speedometer would mean.
A size such as 205/55 R16 means 205 mm wide, a sidewall that is 55 percent of the width, and a 16 inch rim. The overall diameter is the rim plus two sidewalls.
Diameter decides how far the car travels per wheel turn, so a change shows up on the speedometer and the odometer. Width, load index and speed rating are separate questions, so stay within the sizes approved for your car.
Metric sizes only. Check the sizes approved for your car before you buy.
Estimates for general information only, not professional or legal advice. Check your owner's manual, a qualified mechanic and current official rules before relying on a result.
Example
205/55 R16 has a diameter of 631.9 mm. 225/45 R17 has a diameter of 634.3 mm, which is 0.38 percent larger. When the speedometer shows 100, the true speed is about 100.4.
Questions
How much can tire diameter change?ShowHide
There is no single number for every car. Many fitters aim to stay close to the original diameter, but the approved sizes for your car and your local rules decide what is allowed.
Does a different size change my speedometer?ShowHide
Yes, when the diameter changes. A larger tire travels farther per turn, so your true speed is higher than the reading.
Are my numbers saved?ShowHide
No. Nothing is stored when you leave this page.