Tyre Size & Fitment Upsize

Before you spend several hundred on new tyres, find out what they will actually change. Tire Size compares any two sizes and tells you the truth about the swap: the real diameter difference, how much the ride height moves, what happens to your gearing, and how far out your speedometer will read. WHAT IT TELLS YOU Speedometer error, in figures. Your speedometer is calibrated for the size that left the factory. Change the rolling diameter and it lies to you. This shows by how much, at 60 mph for free, and at every speed you drive with Pro. The diameter and sidewall change. In millimetres and in inches, with the percentage difference, so you know whether it clears. Ride height and clearance. How much higher or lower the car sits, and how much closer the tyre gets to the arch. Effective gearing. A taller tyre changes your final drive ratio whether you wanted it to or not. See it before you feel it. Both notations. Metric like 225/45R17 and imperial like 33x12.50R15, in the same comparison. Cars, light trucks and motorcycles. Sizes that fit. Going up a rim size? It lists the sizes that stay within 3% of the one on the car. Saved vehicles (Pro). Keep your stock, winter and upgrade sizes for each car and switch with one tap. Share the comparison with your shop, or with whoever is telling you it will be fine. WHAT IS FREE The full comparison — diameter, height, speedometer error and gearing — is free, and stays free. A single payment adds saved vehicles, up to four sizes side by side, and the full speedometer table. No account. No subscription. No ads. Every calculation runs on the device, so it works in a car park with no signal.