Tyre Size Calculator

A tyre marking like 205/55 R16 looks cryptic but breaks down cleanly. The first number (205) is the section width in millimetres. The second (55) is the aspect ratio, the sidewall height as a percentage of the width; here that means the sidewall is 55% of 205, or about 113 mm tall. The R means radial construction (basically every modern tyre), and the 16 is the wheel diameter in inches. The calculator translates all that into millimetres: overall diameter, circumference, and how that compares to your original tyre.

Why does this matter? If you swap from 205/55 R16 to 225/45 R17, the wheels look better but the rolling diameter changes slightly, and your speedometer reads off because it counts wheel rotations. A 1% diameter difference equals a 1% speedometer error; UK MoT testing allows up to 10% optimistic but no negative reading at all, so most upsizes need careful checking before they are road-legal.

Speedometers are calibrated to a specific tyre rolling diameter. Fit a tyre 2% larger and the speedometer reads 2% slow: 70 mph indicated is actually 71.4 mph. Fit one 2% smaller and it reads 2% fast, which is illegal in the UK because speedometers must never under-read your true speed. The calculator shows the percentage difference between any two tyres so you can spot the issue before you buy.

For winter tyres, going one size narrower with a taller sidewall is common; this gives better grip in snow without breaking speedometer accuracy. For low-profile upgrades, going up an inch in wheel size while dropping aspect ratio (the 'plus one' formula) is the standard way to keep rolling diameter constant. The tyre comparison panel shows you whether your chosen swap stays within tolerance.

Wider tyres (more grip in the dry, often slightly worse in standing water and at slow parking speeds, more road noise). Lower profile (better steering response, more crashy ride, much easier to kerb the alloy). Taller overall diameter (taxes the gearbox slightly, raises ride height, can hit the wheel arch on full lock). Narrower (better in snow, slightly worse braking on dry roads, lower fuel cost from less rolling resistance). The calculator shows you the geometry so you know what you are signing up for before the new tyres go on the car.