Miles per hour to millimeters per second conversion calculator above calculates how many millimeters per second are in 'X' miles per hour (where 'X' is the number of miles per hour to convert to millimeters per second). In order to convert a value from miles per hour to millimeters per second (from mph to mm/s) type the number of mph to be converted to mm/s and then click on the 'convert' button.