I've had one for a couple of years now. It is pretty accurate with ETs. I ran it at the track along side the track timers and it was typically about 1/10 slower than the dragstrip timers. Mostly due to the rollout that you can get before you break the beam at the dragstrip where this device starts the instant the car moves. MPH though is a different story. The g-tech is about 3-4 MPH high in cars that trap 100-110, and 6-7 mph high on cars that trap up to 120. Nobody seems to have an accurate reason as to why, my theory is that if the sample rate of the unit were higher like the more expensive vericom or similar, it would be more accurate with the speed. I mostly use it to learn how to drive the car for best ET and to evaluate any changes that I do to the car like upping boost, fuel, changing parts, etc....