Tire size calculator
Compare two tire sizes to see the difference in overall diameter, and how much it could throw off your speedometer.
Fill in the fields and press Calculate to see your result here.
How this calculation works
Overall tire diameter is the rim diameter plus twice the sidewall height, where sidewall height = width × (aspect ratio ÷ 100). A larger overall diameter means your car travels slightly further per wheel rotation than the speedometer (calibrated for the original size) assumes, so it will under-read your true speed by roughly the percentage difference in diameter.
Worked example
Comparing 205/55R16 to 225/45R17: the second tire's overall diameter (24.97 in vs 24.88 in) is only about 0.4% larger — close enough that your speedometer stays accurate.
Tips
A common rule of thumb is to stay within ±3%
Many tire shops suggest keeping the overall diameter change within about 3% of the original size to avoid noticeably affecting handling, ABS calibration and speedometer accuracy.
How to read a tyre size
In 205/55 R16, 205 is the width in mm, 55 is the sidewall height as a percentage of width and 16 is the rim diameter in inches.
Speedometer error
A tyre 3% larger in diameter makes the speedometer read about 3% low: at a true 100 km/h it shows roughly 97.
Diameter isn't everything
Load index, speed rating and clearance in the wheel arch also matter. Ask a tyre specialist before changing size.
Frequently asked questions
Which tire is Tire 1 and which is Tire 2?
It doesn't matter which is which — the calculator shows the percentage difference between them either way.
Does this account for tire brand differences?
No, it uses the standard size-code formula only. Actual tire diameter can vary slightly between manufacturers even at the same size.