Is a car's speedometer accurate? In a word, no. Here in the 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 reasonably accurate.
Periodically, 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 tires or wheels; even the same size tires can be slightly different diameters if they are different brands.
This a fairly straightforward exercise, but you will need a stopwatch (and luckily 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. While it is possible to do this yourself, it might be a good idea to enlist an assistant so you can drive without distractions.
With the stopwatch or wristwatch, 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 actual 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.
Remember, 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
We can see that the car’s speedometer is reading a little slow, but since it is off by just over 2 mph it’s probably nothing to worry about. It’s when a speedometer is off by 5 mph or more that you would may want to consider having it checked. If the car is modern with an electronically controlled speedometer it may be able to be adjusted at the dealer. Older cars will need a different speedometer drive gear installed in the transmission.