I would think that there has to be some procedure to calibrate the alarm's tach with your stock tach (or something more reliable even). I had a weird problem w/ my turbo timer when I had disconnected the battery and didn't re-calibrate my TT. The timer thought my RPMs and speed were way higher than actual, resulting in 4 minute-plus countdowns. It could be something similar w/ your alarm. At any rate, it would be a good starting point to check out.