Tire Size Calculator

205 is the tread width in millimetres, 55 is the sidewall height as a percentage of that width (so 112.75 mm), R means radial construction, and 16 is the rim diameter in inches. Only these three numbers determine the tyre’s size; letters after them describe load and speed capability.

The widely used limit is ±3 %, which is what the tool checks. Inside that, the speedometer error stays small and ABS and stability systems keep working from sensible assumptions. Beyond it you are into territory where the gauge, the odometer and driver-assistance calibration all drift together.

Bigger tyres make the speedometer read low — you are going faster than it shows. Smaller tyres make it read high. Manufacturers deliberately build in a small over-read from the factory, so a slightly larger tyre can leave you closer to the true speed than the original, which is a reason to check rather than assume.

Yes, by the same percentage as the circumference. A 2 % larger tyre means every 100 km driven registers as about 98 km, which quietly shifts service intervals and makes the trip computer’s l/100 km figure optimistic. Consumption itself changes too, but for physical reasons — mass, rolling resistance and frontal area.

Fitting a bigger rim with a lower-profile tyre so the overall diameter stays the same: one inch more rim usually means about ten points less aspect ratio and ten to twenty millimetres more width. Steering response sharpens, comfort drops, and wheel damage from potholes becomes more likely.

Within limits — each rim width supports a range of tyre widths, typically about 20 mm either side of nominal. This tool covers the geometry, not the rim-fitment table, so check the tyre maker’s recommended rim width as well as clearance in the wheel arch.

Narrower tyres on a smaller rim cut through snow and slush better and cost less, and the taller sidewall survives frozen potholes. That is why manufacturers often list a smaller winter size on the same car — the diameter is kept close so nothing else needs to change.

On the same axle, always. Some cars are designed with a staggered setup (wider at the rear), and that is fine when the manufacturer specifies it. Mixing diameters across an axle, or across axles on a four-wheel-drive car, loads the differential and can damage it.

Some light-truck and van wheels use half-inch rim diameters. The tool accepts them with a dot or a comma, and the geometry works the same way; only the rim conversion changes (17.5 × 25.4 = 444.5 mm).

No, and it is deliberately ignored so the size still parses when you paste a full sidewall marking. Load index and speed rating do matter legally and practically — never go below what the vehicle documentation requires — but they do not change any dimension.

No. It does the geometry; approval is vehicle-specific. The manufacturer’s plate and the registration document list the approved sizes, and an inspection will check against those, not against a percentage.

Because it is the number that connects tyre size to everything the car measures: speed, distance and wheel-speed-based systems. It also lets you compare a tyre against a manufacturer’s published figure, which is usually given as revolutions per kilometre rather than as a circumference.