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.
The size of the error matches the change in diameter. A 2% larger diameter makes the speedometer under-read by about 2%, an indicated 70mph is really around 71.4mph. The odometer drifts the same way, quietly under- or over-counting mileage, which can matter for warranty or lease terms.
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.
From the workshop: this is the part people forget when they fit a taller tyre. The car ends up going faster than the dial says, which is the wrong way round legally and an easy way to catch a speeding ticket. Keeping the rolling diameter close to standard keeps the speedo honest.
Sources and accuracy. The relationship between overall diameter and speedometer reading, and the requirement that a speedometer not under-read, reflect the standards in force at the time of writing. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Does changing tyre size affect the speedometer?+
Yes, if the overall diameter changes. The speedometer is calibrated to the original tyre's revolutions per mile, so a taller or shorter tyre makes it read high or low. The error percentage matches the percentage change in overall diameter.
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?+
By the same percentage as the change in overall diameter. A 2% larger diameter means the speedometer under-reads by about 2%, so an indicated 70mph is really about 71.4mph. This is why size changes are kept within a small tolerance.
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.