Why tyre size changes your speedometer
A speedometer does not measure speed directly. It counts wheel rotations and multiplies by an assumed distance per revolution — the circumference of the tyre the vehicle was calibrated for. Change the tyre's rolling circumference and every reading is wrong by the same proportion.
Larger tyres travel further per rotation, so the vehicle is moving faster than the display shows. Smaller tyres do the reverse. The error is proportional, so it grows with speed: a 3 percent discrepancy is about 2 km/h at 70 and about 4 at 130.
The odometer is affected identically, which has consequences beyond the display. Distance is under-recorded with larger tyres, so service intervals arrive later than they should, fuel economy calculations are wrong, and recorded mileage understates actual use — relevant for leases, warranties and resale.
Reading a tyre size
A marking such as 225/45R17 gives three numbers. 225 is the section width in millimetres. 45 is the aspect ratio — sidewall height as a percentage of width, so the sidewall here is 101.25 mm. 17 is the wheel diameter in inches.
Overall diameter is the wheel diameter plus twice the sidewall height, converting units consistently. This is why aspect ratio matters as much as the other figures: a wider tyre with a lower profile can have the same overall diameter as a narrower one with a taller sidewall, which is the principle behind plus-sizing — fitting a larger wheel with a lower-profile tyre to keep the rolling circumference unchanged.
Rolling circumference is slightly less than the geometric circumference, because a loaded tyre deflects and its effective radius is reduced. Manufacturers publish revolutions per kilometre or mile for this reason, and those figures are more accurate than a calculation from the sidewall markings.
Legal and practical limits
Most jurisdictions require that a speedometer never reads below actual speed, while permitting it to over-read. UK and EU rules under UNECE Regulation 39 allow an indicated speed no lower than true speed and no more than 10 percent plus 4 km/h above it, which is why vehicles typically read a few percent high from the factory. Fitting larger tyres eats that margin and can push the reading below true speed, which is non-compliant.
The general guidance is to stay within about 3 percent of the original rolling circumference. Beyond that, expect meaningful speedometer error, and be aware that anti-lock braking, stability control, traction control and transmission shift points all derive information from wheel speed sensors, so a substantial change affects systems well beyond the display.
There are physical considerations too: clearance against suspension and bodywork at full lock and full compression, load rating, and speed rating — all of which must at least match the manufacturer's specification. Insurance and roadworthiness testing may treat a significant deviation as a modification. This calculator gives a geometric estimate; check your vehicle's handbook and local regulations, and have any change assessed by a qualified fitter.