How the comparison is calculated
Each size is converted to its overall diameter (rim diameter plus two sidewalls). The speedometer error comes from the ratio of the two rolling circumferences, which is the same as the ratio of the diameters:
Actual speed = speedometer reading × (new diameter ÷ old diameter)
Worked example
Going from 225/45R17 (634.3 mm) to 235/40R18 (645.2 mm): ratio = 645.2 ÷ 634.3 = 1.0172. When the speedometer shows 60 mph you are really doing 60 × 1.0172 = 61.0 mph, and at 70 mph about 71.2 mph. The car also sits about 5.5 mm (0.21 in) higher.
Before you change tire size
- Check the wheel width suits the new tire's width, and the load index and speed rating meet your placard.
- Wider or taller tires can rub at full lock or under compression — check the clearance with the wheel offset calculator if the wheels change too.
- On 4WD and AWD vehicles all four tires should have the same rolling circumference.
- In some countries a size not listed in the vehicle papers must be approved or registered (for example in Germany or Switzerland). In the UK and US, tell your insurer about modifications.
Frequently asked questions
How much bigger or smaller can new tires be?
Most tire fitters and manufacturers work to within about ±3 % of the original overall diameter. Beyond that the speedometer error grows, ABS and stability control may be affected and the tire may rub on the bodywork or suspension.
Why does my speedometer read low with bigger tires?
The speedometer counts how fast the wheels turn. A taller tire travels further per turn, so at the same wheel speed the car is going faster than the speedometer shows.
Is a speedometer that reads high legal?
Rules (such as UNECE Regulation 39 in the UK and Europe) allow a speedometer to show more than the true speed but never less. That is why factory speedometers usually read slightly high, and why fitting smaller tires is less risky for speeding than fitting bigger ones.
Does a wider tire change the speedometer?
Only through the diameter. If the overall diameter stays the same — for example 205/55R16 to 225/45R17 — the speedometer is essentially unaffected even though the tire is wider.
Will the odometer be wrong too?
Yes, by the same percentage. With tires 2 % taller, every 100 miles shown on the odometer is really about 102 miles driven.