Tire Size Comparison
A tire size like 225/45-17 is a formula, not a name: 225 mm of width, a sidewall that is 45% of that width, on a 17-inch rim. Put those three numbers in the diameter equation and you can predict everything that matters before the tires are mounted - how much taller the new setup sits, what your speedometer will read when you're actually doing 100, and whether the odometer just started lying to you.
This calculator runs the comparison live and adds the verdict that tire shops argue about: the ±3% rule. Stay inside it and gearing, ABS, ESP and rub clearances stay within what your car was engineered for; drift outside and the note explains exactly which problems you've signed up for. It's the math behind why plus-sizing pairs a bigger rim with a smaller aspect ratio - and why some swaps simply don't work.
How to use
- Enter your current size from the sidewall (e.g. 225/45-17) as the baseline.
- Enter the size you're considering.
- Check the diameter change, speedometer reading and the ±3% fitment verdict.
Frequently asked questions
How do I calculate tire diameter from the size?
Diameter in mm = rim inches × 25.4 + 2 × (width × aspect ÷ 100). For 225/45-17: sidewall is 225 × 0.45 = 101 mm, so diameter = 431.8 + 202.5 ≈ 634 mm. Compare diameters between sizes and everything else - speedo error, clearance, revs per mile - follows from that single ratio.
Will bigger tires mess up my speedometer?
Yes, proportionally: speedo reads a percentage based on rolling diameter, so a +3% taller tire makes it read 97 when you're doing 100. The calculator shows the exact reading at 100 km/h for your swap. Odometer drifts the same way - enough to matter for leases and fuel logs.
What is the ±3% rule?
A practical industry guideline: keep total diameter within about ±3% of the factory size so gearing, braking, ABS/ESP calibration, and body or fender clearance stay within design tolerances. Beyond that you're into rub-on-full-lock territory, speedometer correction, or insurance questions - sometimes all three.
Why do people put bigger rims with thinner tires?
To keep the total diameter constant while filling the arch with more wheel: a plus-one or plus-two swap increases rim inches but steps the aspect ratio down so the sidewall shrinks by the same amount. The math only works if the sum holds steady - which is exactly what this comparison checks before you spend money.