What's a speedometer?
In short, a speedometer is a system that works out how fast your auto or motorcycle is travelling, and displays it on a gauge in front of you.
How accurate is a speedometer?
Your car's speedometer tells lies. Well, when the car is moving anyway. Indeed, if you've ever used a sat-nav app on your phone you may have seen that it shows you're travelling at a slightly different speed than that shown on your car's speedometer.
Here in the United States, a speedo can under-read or over-read by a small margin because its accuracy is mandated by the Society of Automotive Engineers. Over in Europe a speedometer isn't allowed to under-read at all. But wherever your car comes from, the speedo still needs to be pretty accurate.
That's why it’s a good idea to check the accuracy of your speedometer every so often, just to be sure that what it’s showing you is reasonably close to the truth and you're not breaking any speed limits. Doing this makes real sense if you've just fitted new tires or wheels; even tires of the same size can be slightly different diameters if they are from different brands.
How do I check my speedometer?
This is easy, but you'll need a stopwatch (most smartphones have one built in) and a 5- to 10-mile stretch of highway where there are mile markers along the shoulder. You can use nearly any freeway, or limited access expressway, or even a less-traveled state or county road.
We'd also strongly recommend that you take along someone to operate the timer, so you can drive without distractions.
Just time your vehicle for several miles over a stretch of road at a steady speed (If your car has cruise control, use it). To determine your speed, 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 traveled.
How do I work out my speedometer’s true reading?
If you wind up with, say, 5 minutes and 15 seconds for 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. You may now plug this figure into the following equation:
Actual MPH = 60 / minutes x number of miles traveled
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. In this particular case, 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 speedo 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.