You shouldn't have to input an accurate weight to get accurate acceleration times. The weight input is only needed for accurate calcuation of HP. Otherwise, the accelerometers in these devices use g-forces and time to determine 0-60 and 1/4 mile times - that's all they need. Xg for x.xx seconds will determine all that because weight does not factor in. For example, 3g is 3g no matter if you're in 2,000lb car or 5,000lb car. But it takes a lot more HP to sustain that 3g in a 5,000lb car than it does in a 2,000lb car, so if you want an accurate HP calculation done, you need an accurate weight.

I have a similar device, a Belltronics Vector, and it always gave me times very close to my time slips at the track, usually within a tenth of what the time slip read for a given run (yeah, I'd have the thing running at the strip).

You will notice a couple of minor differences between one of these and a track time slip, for a couple of reasons:

1. The track time slip is based off the timing lights, obviously, which means it doesn't start timing until you've broken out of BOTH staging lights so it can vary just a touch depending on whether you deep stage or not. An accelerometer starts timing the instant the vehicles moves
2. The track time slip trap speeds are calculated from the two sets of trap lights at the top end of the track - normally 20 meters or 66 feet apart. The accelerometer gives you a "trap speed" calculated at the very end of the 1/4 mile, so if you're still pulling hard in those last 20 meters the accelerometer will show a higher trap speed than the official time slip will.