[MustangDanny]

G-Techs are absolutely accurate +/- 2 mph and about a tenth or so in the ET. They work based on an inertia sensor. If you have the inertia data from anything you can tell it's speed, acceleration and evern "jerk" which is the rate of change of acceleration. Basically it has some basic computing power to crunch some of the numbers and spit out ET's and MPH's. The HP reading it puts out requires you to input the cars weight, then it just uses the acceleration per weight figures and converts them to the hp units.

Can beat it for tunning. Really clears up the picture on what really makes a difference and what doesn't (i.e changing fuel pressure, adding a small bolt on, etc.).