| I have a GTech/Pro and can't comment on the accuracy compared with a dyno. However, I would be very surprised if it matched the dyno as the GTech does not take into account the power needed to overcome air resistance. I get readings that are typically 30hp less than I others are getting on the dyno and consider this reasonable - I usually run the tests in 2nd gear and max hp comes close to 60mph. 30hp is about right for the power needed to push the car through the air at that speed.
I consider the GTech to be a great tool for comparing before and after results from mods but it will never give absolute power numbers accurately.
2007 DS/W MCS LSD, sport suspension, Enkei RPM2 wheels
2002 EB/W MCS Sport package, Rogue intake, Borla exhaust, 15% pulley & Madness sway bar | 
