Load up a Single player, Custom game, with the following settings:
Map type: Inland
Map size: Tiny
Reveal map: Normal
All the other settings don't really matter. Then once the match starts, press ENTER, type " bigdaddy " and press ENTER again to spawn a race car. Simply move this car around the map to reveal all of it, which will take less than a minute. This will unlock the achievement for you.