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