Ask of the Week: Which is more accurate - my speedometer or my sat nav?

Dear HonestJohn,

"What is the most accurate speed indicator - analogue speedometer, digital speedometer or GPS on sat navs?"

- LE

Dear LE,

Vehicle speedometers, whether digital or analogue, calculate speed based on the rotation of the wheels or drivetrain. Accuracy can be affected by differences in tyre tread depth, wheel sizes and even tyre pressures.

By law, vehicle speedometers must not under-read nor show more than 110% of your actual speed plus 6.25mph, so they are designed to always over-read by a margin in order to never under-read. This means at a true 70mph your vehicle could be indicating 83.25mph and still be within the vehicle regulations.

Satellite navigation uses GPS to locate your position and can also provide your speed by constantly calculating how long it takes you to cover the distance travelled. Although not necessarily 100% accurate, GPS-calculated speed will usually be more accurate that the vehicle speedometer.

Professional speed testing equipment also uses GPS to provide highly accurate data, but this is done by measuring the Doppler shift in signals from a number of satellites.

Ask HJ

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?
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.
Answered by David Ross
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