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 traveledminutes
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 55 mph. 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 your speedometer checked or have its drive gear replaced.