To get this achievement you have to finish a race in 108 seconds or 1 minutes 48 seconds. It doesn't matter if it is 1:48.4 or so. As long as it reads 1:48, it will work. An easy way to do this is on the track "Restoration Area" in the "North America Tour Part 1." You can just take the track slow and stop before the finish line, then when the clock reads 1:48, pass the finish line.