Changing tyre size affects the speedometer whenever it changes the tyre's overall diameter. The speedometer counts wheel revolutions and is calibrated to the original tyre, so a taller or shorter tyre makes its reading drift, and the direction matters.
Why it happens
A speedometer works from how many times the wheel turns, converted to a speed using the original tyre's rolling distance per revolution. Change the overall diameter and each revolution now covers a different distance, but the speedometer still assumes the old one. The reading is wrong by the same proportion the diameter changed.
Which way it reads
The direction is the important part:
- A larger overall diameter travels further per turn, so the car is actually going faster than the speedometer shows. The speedometer under-reads.
- A smaller overall diameter travels less per turn, so the car is going slower than shown. The speedometer over-reads.
A larger rolling circumference increases distance travelled per revolution relative to the original. The dashboard reading also depends on its existing calibration, so a dimensional calculation is not an exact speed prediction.
Why under-reading is the problem
There is a legal angle. A speedometer is required to never read less than the true speed, it may read a little high, but not low. A larger tyre, by making the speedometer under-read, pushes it in the one direction the law does not allow, and risks the driver unknowingly speeding.
This is a core reason size changes are kept within a small tolerance, usually within about 3% of the original diameter. Plus sizing sidesteps the issue entirely by holding the overall diameter steady while only the wheel and profile change. Keeping any size change within that tolerance is what keeps the speedometer reading legal, and comparing the right fitments by size at a tyre retailer such as Tyres.co.uk is the easy way to stay within it.
Use the calculation as a comparison
Compare the proposed overall diameter with the original and express the difference as a percentage. That percentage estimates the change in distance per revolution, not the exact speed the dashboard will display. Do not treat a phone speed reading as approval of a new fitment. The replacement must also satisfy the vehicle's load, wheel and clearance requirements, and the speedometer must remain compliant. A tyre that fits under the wheelarch is not necessarily a suitable conversion.
Common questions
Does changing tyre size affect the speedometer?+
The change in nominal circumference estimates the change relative to the original tyre. The exact indicated speed also depends on the original calibration and actual tyre dimensions; a calculator cannot establish a compliant fitment.
Do bigger tyres make the speedometer read faster or slower?+
Slower than reality. A larger overall diameter travels further per turn, so the car is actually going faster than the speedometer shows, it under-reads. A smaller diameter does the opposite and makes the speedometer over-read.
How much does tyre size change the speedometer?+
The change in nominal circumference estimates the change relative to the original tyre. The exact indicated speed also depends on the original calibration and actual tyre dimensions; a calculator cannot establish a compliant fitment.
Is an inaccurate speedometer from bigger tyres legal?+
A speedometer must never read less than the true speed. A larger tyre that makes it under-read pushes it the wrong way and can take it outside legal calibration, as well as the obvious risk of unknowingly speeding. Keeping the diameter close to standard avoids this.