If calibrated correctly they can be fairly accurate as far as the times that they report (0-60 and 1/4 mile). Again, this depends on being able to launch the car with NO wheelspin whatsoever. Which is going to make you slower. But yes, set up properly, I believe it can come within a couple tenths of your actual times.
The HP and TQ numbers it gives are significantly lower than you will get on any dyno.
The car I owned when I tried the G Tech had been on a few different dynos, and consistently put down in the 230-240 whp range at the time. After several runs with the Gtech, the highest HP I registered was 202, if I remember correctly, or may have been 212. They SAY it's your 'true, aerodynamics limited HP'. *shrug*. I say, the HP results were not consistent enough to really mean anything and I wouldn't even think of trying to tune on that thing.
