ORIGINAL: bscottie
I'm glad you found this post, the accuracy however depends on the accuracy of the input data for your specific parameters, I did as the book said, the article and manufacturor guarantee the accuracy to certain percentages, the only variables that I have uncertanty about are pitch and weight of vehicle, I like the little gadget, and actually believe that with some time I could get it to do just as advertised, I still dont believe that I'm pushing 253hp and 264# trq, but I will get to a closer point of actuality with this "cool gadget" in the future. It is still just for fun.

ORIGINAL: Mike 83 GLX
....The most important thing with these is that you must know the exact weight of the vehicle and everything inside it as-tested, in order to calculate horsepower correctly. And the road must be perfectly flat and level for the accelerometer to work with minimal error. I always see people get strange numbers because they read somewhere that their car weighs XXXX lbs so that's all they enter. I've weighed my Mustang before testing and my results have been fairly accurate.

If nothing else they're consistent, and good enough to use to find out if you gained anything from your mods. When I used it at the track, my 1/4 times were within .1 sec and a couple mph, but the speed is calculated differently than the track timers. I'll try to remember to dust mine off and test it again when I get dyno tested this summer.