The reason the mph differs between the track and the G-Tech is because the track does not actually give you your 1/4 top end speed. They time you between two points, (a 66 foot speed trap), in the shutdown area and give you an average speed between those two points. Typically, you are still accellerating, since you haven't crossed the last stripe yet.

The G-Tech is going through a big calculation, actually giving you a speed that is reached at the end of the 1/4 mile.

I would be interested to see what the results would be if someone were to use a radar gun on a car at the track, so you could compare the G-Tech to the track to the radar gun.

I like my Pro Comp. It's very accurate.
Within a few tenths.