Tire Size Calculator
Compares two tyre sizes from their sidewall codes. The difference in rolling diameter is what changes your speedometer reading, your gearing and your ground clearance, so it is the number that matters when changing size.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
diameter = rim × 25.4 + 2 × (width × aspect ÷ 100) speedo error = (new diameter − old diameter) ÷ old diameter
The aspect ratio is a percentage of the tyre width, not a fixed measurement — which is why a wider tyre at the same aspect ratio is also a taller one. That catches people out when they widen a tyre and expect the diameter to stay put.
What to watch out for
Reading a sidewall code — 225/45R17:
- 225 — section width in millimetres.
- 45 — aspect ratio: the sidewall height as a percentage of the width. Here 45% of 225 mm, so 101 mm.
- R17 — radial construction on a 17-inch rim.
Because the aspect ratio is a percentage, going wider at the same profile also goes taller. 225/45R17 and 245/45R17 differ by 18 mm in diameter, not zero.
Stay within about 3%. Beyond that the speedometer error becomes legally significant in many places, and the tyre may foul the arch under compression or at full lock.
Frequently Asked Questions
How do I read a tyre size?
225/45R17 means 225 mm wide, a sidewall 45% of that width (101 mm), radial construction, on a 17-inch rim. The overall diameter is the rim plus two sidewalls.
How much does tyre size affect the speedometer?
Directly and proportionally. A tyre 2% larger in diameter means your true speed is 2% higher than the dial shows — at an indicated 70 mph you are actually doing about 71.4.
How much bigger can I go?
Around 3% in diameter is the usual guideline. Most speedometers are calibrated to read slightly high from the factory, so a small increase brings them closer to true — but past 3% you risk arch clearance and ABS calibration.
Does a wider tyre have a bigger diameter?
Yes, if the aspect ratio stays the same, because the sidewall is a percentage of the width. To go wider without going taller you have to drop the aspect ratio — which is exactly what plus-sizing does.