Some Tweeking would make it awesome.
I love the idea and it works decently well. there are a couple of improvements I would recommend. The key improvement that you could do is first to change the sound that tells you to fire. I managed fire almost exactly when the alarm sounded several times and wound up with seemingly random results. This might be that the sound and the timer aren't exactly synchronized or that the sound is just so ... umm... unpointed that it is difficult to nail down when it starts. Maybe use a sound that has a hard, loud, and very definitive start. After all, races start with a gunshot.
I'm also a little confused as to whether or not it counts the time from the sound to the shot or just over time in general.
That's all technicalities though. the game was great, it would have been even greater if it were bigger, with more little minigames or more levels, but in general, woot. fun! and adictive! Fived.