The GTech/Pro uses a linear accelerometer to measure fore and aft acceleration. Side to side acceleration is measured by turning the unit 90 degrees. Using values for acceleration and basic physics, it can determine both velocity and distance traveled. This is how it is able to return 0-60 times and 1/4 times without being hooked into the vehicle speedometer. It's actually similar to the guidance systems of some missles which measure acceleration to figure out where they are, and adjust accordingly.
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There are two differences between drag strip timing and GTech acclerometer measurement. The first difference is at the starting line. The 1/4 mile timing equipment allows a rolling start. The GTech is an instant start when the car moves the time starts. The second, as Shawn stated, is the trap speed. The 1/4 mile timing equipment calculates mph over the last 66ft of the 1/4 and Gtech figures the instant mph at the finish line.
Out of curiosity we used a Vericom (same principle at GTech) at the drag strip. As a rule of thumb the Vericom produced a slower et than the timing equipment. The et difference in the instant start of the Vericom compared to the rolling start of timing equipment could be as much as 2/10 depending on the launch of the car and how shallow the car was staged.
The mph of the Vericom was higher than that of the timing equipment. The difference was dependant on the car. Cars that were still accelerating hard had a larger difference than those that had peaked and the acceleration had leveled off.
The GTech/Vericom accelerometer type measurement in itself is accurate. Any error is mostly user induced. ie The meter not correctly mounted. The meter not being placed in the exact same location from the previous time. Road texture, hills bumps, or an incline or a decline. Incorrect information such a vehicle weight entered into the meter.
When correctly used the GTech can be a valuable tool in measuring differences in performance/vehicle changes.
