If I change tyre size will this make the speedometer inaccurate?

If the smaller of the two numbers on a tyre relates to height will remaining on the same size wheel with taller tyre not affect speedo readings?

Asked on 16 October 2023 by David Taylor

Answered by David Ross
The size label on a tyre indicates the tyre width, its sidewall height as a percentage of its width and finally the wheel rim size it is designed for. As an example a 205/50R16 tyre is 205mm wide, an aspect ratio of 50% which would be 102.5mm and would be suitable for a 16-inch wheel.

Increasing the aspect ratio to 55 would give a sidewall of 112.75mm, an increase of 10.75mm. At an indicated 60mph this would give an actual speed of 62mph, and at an indicated 100mph this would give an actual speed of 103mph.
Similar questions
How accurate are car speedometers and what is the effect on accuracy of changing running radius of tyre as tyres wear?
In 2022 you advised that it would be possible to replace my low-profile wheels with smaller wheels and thicker tyres. When I asked Kia, they refused to sell me new wheels as they said that this wouldn’t...
What is the most accurate speed indicator - analogue speedometer, digital speedometer or GPS on sat navs?

Value my car

Save £75 on Warranty using code HJ75

with MotorEasy

Get a warranty quote

Save 12% on GAP Insurance

Use HJ21 to save on an ALA policy

See offer