There are 1. 609344 kilometers in one mile. Therefore, if you know the speed in miles per hour, just multiply it by 1. 609344. This will give you a value that is probably more accurate than you need, so you will need to round. Here's an example: 25 miles per hour is 40. 2336 kilometers per hour because 25 X 1. 609344 = 40. 2336.