Look at the box or disc.
If it says NTSC-J, you have the Japanese/Asian version.
It it says NTSC-U/C, you have the North American version.
If it says PAL, you have the European/Worldwide version.
Only the NTSC-U/C version used mph, so it's highly likely that you have the NTSC-J or PAL versions. That aside, if you're playing them on PS2, each will only play on the correct console - you can't play an NTSC-J disc in a PAL console - so it's highly likely that your game version will match where you are. As your profile says you're in the UK, it's most likely you have the PAL version.
It is possible to recode a gamesave from NTSC-U/C to work with a PAL disc. This will give you a speed displayed in mph, but it won't give you anything else. You will also lose all of your game progress, as it won't be your gamesave. I don't even remember where my tools are for doing this, and I don't have the program on this PC either. There may be a blank recoded save floating around the forum somewhere.
