Divide the original speed in miles per hour by 60. This converts mph into miles per minutes (because there are 60 minutes in an hour). For example, if a car is traveling 90 miles per hour, you would divide 90 by 60 and get 1.5 miles per minute.
Divide the miles per minute calculation by 60. This will convert miles per minute into miles per second (60 seconds in a minute). Using the example in Step 1, you would divide the 1.5 miles per minute by 60, giving you 0.025 miles per second.
Check your work by multiplying the miles per second number by 3,600 (60 seconds times 60 minutes). The answer to this equation should be the original speed that you were working with. For future problems, you can simply divide the original mph reading by 3,600 to get miles per second, but it is important that you understand where the 3,600 number comes from.