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It's all in how you set it up(perfectly level,plumb etc).
I tested it at the track and was off by a few tenths and several MPH, butteh G-Tech will always show a higher MPH due to the fact that it gives terminal speed rather than trap speed.
To use as a tuning tool to gauge improvement, it's not bad, but I wouldn't rely on it for accurate 1/4 mile times.
I haven't tried the newer model which I hear is supposed to be more accurate.
I tried one a week or so ago on mine. No N20 used and I spun thru 1st and half way thru 2nd and she read 14.2 @ 105mph. The last time I went to the strip I ran 13.8's @ like 97mph with no spin and no N20. So I would say that the times are close and the speed is a little high (like Mojo said)
The ETs from the G-Tech will be SLIGHTLY slow, depending on how you stage at the strip. The G-Tech starts timing when it senses movement. That is consistent with a deep stage. In a shallow stage, you get a "running start" at the starting line clocks. Eliminate your wheel spin. Your G-Tech ETs are close to what you should expect.