How to Use This Tool
Enter your current size and the one you are considering. The important output is the diameter change, because everything else follows from it.
Reading the code
In 225/45R17:
- 225 is the section width in millimetres — the widest part of the tyre, not the tread.
- 45 is the aspect ratio: the sidewall height as a percentage of the width. It is not a measurement, which is the part that catches people out.
- R means radial construction.
- 17 is the wheel diameter in inches.
So the sidewall is 225 × 0.45 = 101.25 mm, and the overall diameter is the wheel plus two sidewalls: 17 × 25.4 + 2 × 101.25 = 634.3 mm.
Because the aspect ratio is a percentage, narrowing the tyre also lowers the sidewall. Going from 225/45 to 205/45 loses 9 mm of sidewall on each side without the "45" changing at all — which is why plus-sizing charts drop the aspect ratio as the wheel grows.
Why the speedometer goes wrong
A speedometer counts wheel rotations and multiplies by an assumed circumference. Fit a taller tyre and each rotation covers more ground, so the real speed is higher than the reading. Fit a shorter one and it reads high.
225/45R17 to 245/40R18 is 2.98% taller. An indicated 60 is really 61.8, and an odometer showing 1,000 miles has actually covered 1,029.8. The odometer error compounds over the life of the car and quietly brings forward every service interval.
Reading high is the safe direction, and most jurisdictions require it: regulations typically allow a speedometer to over-read but never to under-read. Manufacturers therefore set them a few per cent optimistic from the factory, which means a taller tyre eats into a margin that was already there for a reason.
The 3% rule
The usual guidance is to stay within about 3% of the original diameter. It is a convention rather than a law, and the reasoning is sound:
- Speedometer accuracy is already using part of its legal margin.
- ABS and stability control compare wheel speeds against expected values. Small errors are absorbed; large ones are not, and mismatched sizes across an axle are worse than a uniform change.
- Clearance. Diameter changes split evenly, so a 20 mm larger tyre adds only 10 mm at the top — but that is 10 mm closer to the arch liner at full lock and full compression, which is where rubbing happens.
- Gearing. A taller tyre raises the effective final drive, which softens acceleration and lowers cruising revs slightly.
What this cannot check
Diameter is only one of the constraints. Load index and speed rating must at least match the original, the wheel width has to suit the tyre width, and offset determines whether the wheel clears the suspension and the arch. None of that is visible from the size code alone.
Check the placard in the door jamb for the specified sizes and ratings, and confirm fitment with a tyre fitter before buying. This is arithmetic, not fitment advice.
