Enter both tyre sizes as width / aspect ratio / rim, e.g. 205/55R16.
Advanced options
Convention is to stay within about 2% of the original overall diameter (some say 3%) so the speedometer, ABS and gearing stay accurate. Check load and speed ratings too, and tell your insurer about wheel or tyre size changes.
What The Tyre Size Calculator Works Out
- Diameters: a marking like 205/55R16 means 205mm of width, a sidewall 55% of that width tall, on a 16-inch rim. Overall diameter is the rim (converted to mm at 25.4 per inch) plus two sidewalls: 632mm for 205/55R16, which is what the comparison actually cares about.
- Size difference: the new diameter as a percentage change on the original. 225/45R17 comes out about +0.4% against 205/55R16, which is exactly how plus-sizing is meant to work: rim up an inch, aspect ratio down a step, diameter preserved. Convention is to stay within 2% (some say 3%).
- True speed at a given speedo reading: the speedometer and odometer are calibrated to the original rolling circumference, so a larger tyre makes the speedo under-read (true speed higher than shown) and a smaller one over-read. UK speedometers must never under-read and are usually set slightly high from the factory, so a small increase often just makes it more accurate.
- Speedo reading to test and units (advanced): check any speed, in mph or km/h, instead of the default 70. The second speed card checks 30 (or 50 km/h) for town limits where a few percent matters most.
- Diameter tolerance (advanced): change the 2% verdict threshold to 3% or to a stricter figure if your car's manual or a 4x4's transfer box demands it.
- Sidewall height (advanced): width multiplied by aspect ratio for each tyre. Lower sidewalls sharpen steering response but ride harder, transmit more noise and are easier to damage on potholes and kerbs.
- Circumference and revs per mile (advanced): pi times diameter, and how many times each tyre turns in a mile. A bigger tyre turns fewer times, which is effectively taller gearing: slightly lower revs at cruise, slightly blunter acceleration.
- Radius change (advanced): half the diameter change is how much the hub rises or falls and how much less clearance there is to the arch, with the width change per side alongside. Wider tyres can foul arch liners or suspension on full lock or compression.
- Odometer error per year (advanced): how many miles the odometer gains or loses on your annual mileage. A 3% larger tyre on 10,000 real miles records about 290 miles too few, which matters for service intervals, mileage-limited finance and resale.
- Beyond geometry: the new tyre's load index and speed rating must meet or exceed the original (MOT testers and insurers care), mixing different circumferences across an axle or between axles on a 4x4 causes handling and drivetrain problems, and wheel or tyre size changes count as modifications for insurance, so declare them.