Understanding tire size, diameter and speedometer error
How to read a P-metric tire size, why overall diameter drives speedometer accuracy, and how to compare two sizes before you buy.
Breaking down a P-metric size
A modern passenger tire is stamped with three numbers such as 225/45R17. The first, 225, is the section width in millimetres measured across the widest part of the tread. The second, 45, is the aspect ratio, meaning the sidewall stands 45 percent as tall as the tire is wide. The R shows the tire is radial, and 17 is the wheel it mounts on measured in inches. Mixing metric width with an inch rim is why a calculator is handy: you cannot just add the numbers together.
Why overall diameter is the number that matters
Overall diameter is the true height of the mounted tire from ground to top, and almost every downstream effect flows from it. The sidewall height in millimetres is width times aspect ratio divided by 100, then two of those sidewalls plus the rim, all in inches, give the diameter. For 225/45R17 the sidewall is 101.25 mm and the diameter is 24.97 inches. Change the width, aspect ratio or rim and the diameter shifts, which is exactly what alters your rolling distance, gearing feel and speedometer.
How diameter turns into revs per mile and speed
Multiply the diameter by pi to get circumference, the distance covered in one rotation. Divide 63,360 inches in a mile by that circumference and you have revolutions per mile, about 807.6 for a 225/45R17. Your speedometer is really a rotation counter tuned to a factory revs-per-mile figure. Fit a tire that turns fewer times per mile and the gauge reads low; fit one that turns more and it reads high. The percentage the compare box shows is the direct measure of that offset.
Comparing two sizes before you buy
Tick the compare box and enter a candidate size next to your current one to see both diameters and the speedometer error at a glance. A jump from 225/45R17 to 235/55R17 grows the diameter to 27.18 inches, an +8.83 percent change that would make your speedometer read noticeably slow. Staying inside roughly 3 percent keeps the swap close to factory behaviour, but always confirm the new width suits your rim, the load and speed ratings meet or beat the originals, and there is room in the wheel well before committing.