In the United States and some other parts of the world, automobile speedometers provide speed in miles per hour. While this helps you keep track of your speed relative to a posted speed limit, this may not give you a useful estimate of the distance you travel when reacting to immediate events on the road. Therefore, you may wish to convert miles per hour into feet per second to gauge a safe following distance. In order to do this, you must know how many feet are in a mile and how many seconds are in an hour.
TL;DR (Too Long; Didn't Read)
To convert a speed value from miles per hour to feet per second, multiply it by 5,280, then divide by 3,600.
Convert Miles to Feet
Multiply your speed in miles per hour by 5,280. This is the number of feet in a mile. The result is your speed in feet per hour. For example, 60 miles per hour times 5,280 feet per mile is 316,800 feet per hour.
Convert to Feet per Minute
Divide your speed in feet per hour by 60. This is the number of minutes in an hour. Therefore, the result is your speed in feet per minute. For example, 316,800 feet per hour divided by 60 minutes per hour is 5,280 feet per minute.
Convert to Feet per Second
Divide your speed in feet per minute by 60. This is the number of seconds in a minute. Therefore, the result is your speed in feet per second. For example, 5,280 feet per minute divided by 60 seconds in a minute equals 88 feet per second.