How to Compare a Speedometer to a GPS

GPS units use satellite information to locate your position on earth. Because speed is a simple function of distance and time, most GPS units can also find your speed by calculating the distance you travel in a given time. Though it uses a different method, the speedometer in your car uses the same principles for determining speed. Because they use different methods of measurement, GPS and speedometer readings can vary greatly. To find out how they compare, consider testing them against a standard mileage-speed test.

• 1

  Find an area of highway that offers a speedometer test. Speedometer test sections are simply areas on highways that have very specific mileage markers that you can use to check your odometer and speedometer.

• 2

  Accelerate to a safe speed and set the cruise control. If you don't have cruise control, maintain your speed as accurately as possible. When you reach the first mile marker, start your timer.

• 3

  Record the speed that your GPS is reading and the speed that your speedometer indicates. Have a friend read this information so that you can focus on the road.

• 4

  Stop the timer at the last mile marker and record the number of miles you traveled and the amount of time it took.

• 5

  Divide miles traveled by time to get your actual speed using a calculator and the equation below. Enter time in hours; for example, if you drove for 15 minutes, input .25 hours.

  Miles traveled / Time = Speed

  For example, a car that travels 10 miles in 15 minutes would have a speed of 40mph. (10/.25 = 40)

• 6

  Compare your actual speed with your recorded speedometer readings and your GPS speed readings. If either your speedometer or your GPS is off, consider calibrating for more accurate readings in the future.