How far you go in a given time depends on your speed. In the United States, MPH (miles per hour) measures the speed of a vehicle. Using the rate of miles divided by the hours for the speed, you need only the time you traveled to calculate your distance.Use this formula for any type of vehicle  cars, planes or boats  to determine your distance from the speed.
Things You'll Need
 Stopwatch
 Speedometer

Note the speed in miles per hour (MPH) by looking at the speedometer; maintain that speed as you travel, ideally with cruise control engaged. Start the stopwatch.

After the desired period, stop the stopwatch and record the time elapsed.

Convert the time traveled to hours by dividing the number of minutes by 60. For instance, if you traveled for 5 minutes that equals 5/60 = 0.83 hours.

Multiply the number of hours by the speed in miles per hour to find the distance traveled. For example, if you traveled 0.83 hours at a speed of 30 MPH, your distance would be 30 x 0.83 = 2.5 miles.
References
 Photo Credit speed image by kuhar from Fotolia.com