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Speedometer readings: how accurate are they?

A car speedometer

Car speedometers exaggerate – or do they? We investigate.

We've all heard the stories that speedometers say you're going faster than you actually are, but can that really be true? A car's speedometer should be accurate, you'd think, so what are the legal requirements around speedometers, and how can you check yours? Our experts have taken a look.

Can a speedometer be wrong?

Your car's speedometer rarely tells the truth. If you've ever used a sat-nav app on your phone you may have noticed that the speed it shows you're travelling at is slightly higher than that shown on your car's speedometer – and the faster you go the more inaccurate it tends to be. Why is that?

Here in the UK, a speedometer isn't allowed to under-read. Over in the United States a speedo can under-read or over-read by a small margin. But wherever your car comes from, the speedometer still needs to be reasonably accurate, although it's unlikely to be as precise as your phone's GPS, which uses satellites to pin-point its position.

It’s a good idea to check the accuracy of your car’s speedometer, just to be sure that what it’s showing you is the truth and you're not breaking any speed limits. This is especially useful after getting new tyres or aftermarket wheels, especially if the latter are a different diameter to the ones the car had when new.

How do I check my car speedometer’s accuracy?

Checking the accuracy of your speedometer is a fairly straightforward exercise, and you could use said phone's sat-nav speed readout to do this. But it's better to take a more scientific approach.

For this you'll need a stopwatch and a 5-mile stretch of straight motorway; use a book of maps before you get in the car to measure the length of the road you're going to use – or use Google's 'Measure distance' feature on Maps (hence the need for a straight stretch of road).

You'll also need an assistant to use the stopwatch while you concentrate on the driving (and your speedo). Speaking of which, it's best to do this when the road is quiet (the dead of night) because congestion will ruin your calculations.

Get your passenger to start and stop the stopwatch over the set distance while you maintain a steady speed (If your car has cruise control use it because you'll need to maintain the same speed for the duration of the test distance). Then you divide the number of minutes it takes you to drive the distance into 60 (the number of minutes in an hour) and then multiply this figure by the distance you travelled.

A car driving on a UK motorway

How do I work out my speedometer’s true reading?

If you take 5 minutes and 15 seconds to cover a 5-mile stretch, you’ll need to convert the seconds portion into equivalent minutes by dividing by 60. Therefore, 5 minutes and 15 seconds would be 5 + 15/60 minutes, or 5.25 minutes.

Now put this figure into the following equation:

Actual MPH = 60 / minutes x number of miles travelled

If the speedometer reading is higher than the actual speed, the speedometer is fast. If the speedometer reading is lower than the actual speed, the speedometer is slow – this is what you should end up with.

In this example, let’s say that your speedometer was reading 55mph. Doing the calculations using the above equation:

Actual MPH = 60 / 5.25 x 5

Actual MPH = 57.1

Should I worry if my speedometer is wrong?

We can see from the above calculation that the car’s speedometer is reading a little slow – by just over 2 mph – which is to be expected. It’s when a speedometer is off by 5 mph or more that you may want to consider having it checked.

If the car is modern, with an electronically controlled speedometer, your dealer may be able to adjust one of the car's brains. But older models may need a different speedometer drive gear installed in the transmission.