Really, it doesn't matter at all if it's right on or not. As long as it's consistant. Make a few runs with one, get a good base line for it(which requires you to go to a track and compare with known times) and see, on average, how far off if at all it is. Then you can start to play with it.
Does anyone know how reliable it is on the horsepower reading? I'd like to say that it's within 2-5HP, but you know these things are. Also, is the measurement rear wheel, or does it calculate flywheel HP? Eitherway, it should be fun when mine comes in. (I bought some BFG KDWS, for the Black GT)
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The Black Mamba 11.2@124
1994 Cobra
'00 Focus ZTS- K&N drop in. 10.66@66mph 1/8mile