If your speedometer isn't working, every time you drive your car you run the risk of getting a speeding ticket. This is a fact even if you have a radar detector. If it is working and yet isn't registering the correct speed you are still in danger. Here's how to check your actual speed and know if your speedometer is reading correctly.
Find your auto's manual. Check the manufacturer's recommended tire size. The car's equipment is based on this specification. The speedometer is measuring revolutions of the tire per unit time. This is then translated by the speedometer cable to the speedometer.
Check your vehicle's tire size. If the tire size is different from the manufacturer's recommendation, your speedometer may be reading too fast or too slow.
Find a road where the speed limit is at least 60 mph. Most of the time this will be on a freeway or interstate highway. You will need to find a time of day or night when the traffic load of the road is the least congested.
Get on the road at the selected time and ensure that you can maintain a speed of 60 mph. Once you have reached 60 mph, watch for a mile marker. On most interstate highways, there are markers every mile. As you pass one of the markers. note the mile number and start your stopwatch. Continue driving steadily at 60 mph until you reach the next mile marker and stop the stopwatch.
Note the number of minutes and seconds that it required you to drive 1 mile. If it took you less than 1 minute to travel this distance then your speedometer is reading a speed less than what you are actually traveling at. If the time it required you to drive the 1 mile is more than 1 minute, then the speedometer is registering a speed higher than the speed you are actually driving at. To double-check this test, repeat the test but instead of 1 mile, measure the time to drive 10 miles. This will give you an average of ten 1-mile increments.
