Tire Size Calculator
Compare two tyre sizes: diameter, circumference and how much the speedometer reading changes
Loading the tool…
Works out the overall diameter, circumference and sidewall height of a tyre from the size printed on its sidewall, and compares two sizes. Use it before changing to a different tyre or wheel size to see how much the speedometer, odometer and ride height change.
How to use
- Type the size of your current tyre into "Current tyre size (A)", for example 205/55R16.
- Type the size you are considering into "New tyre size to compare (B)". The diameter difference and the real speed at your speedometer reading appear at once.
- Read the "Size comparison" table and the drawing to see how diameter, width and sidewall height change.
- Under "Sizes with almost the same diameter on other rims", press "Compare as B" next to a size to load it as B.
How it works
- Overall diameter = rim diameter × 25.4 + 2 × width × aspect ratio ÷ 100 (mm). Inch sizes such as 31x10.50R15 give the diameter directly.
- The speedometer counts turns of the original wheel, so actual speed = reading × diameter B ÷ diameter A. A larger tyre makes the speedometer read low.
- The warning threshold is a value you choose; it is not a legal or manufacturer limit. Load index, speed rating, pressure, clearance and legality are not covered.
Questions and answers
- How do I read a tyre size like 205/55R16?
- 205 is the section width in mm, 55 means the sidewall is 55% of that width (112.75 mm), R means radial construction and 16 is the rim diameter in inches. The overall diameter is about 632 mm.
- How much does a bigger tyre change the speedometer?
- By the same percentage as the diameter. With a tyre 3% larger in diameter, a reading of 100 km/h means you are actually driving about 103 km/h.
- What is a plus-one tyre size?
- A size on a rim one inch larger with a lower aspect ratio, so the overall diameter stays nearly the same. For 205/55R16 a common plus-one size is 225/45R17, which is 0.38% larger.
- Does a larger tyre raise the car?
- The axle rises by half the diameter difference. A tyre 20 mm larger in diameter lifts the axle by about 10 mm.