On the ET it is pretty accurate +or- a tenth.
The mph is about 3mph high.
It got my ET pretty much dead on, I dont know if the mph is off on all cars but it reads mine about 3 mph high compared to the track.
Actual ET 13.7 @100mph
G-tech ET 13.66@103mph
I am in the same boat as you I just bought a house and it is putting a huge financial strain on me. Somehow I still found the cash for slicks.
Just remember to make sure the G-tech is level and find a straight level street.
Good luck
Later,