With the Gtech it is important to input the correct test weight of the car (including driver). The GTech will also slightly underestimate the rwhp due to drag which is not a factor on the dyno. This is why they reommend that you do the GTech hp test in a lower gear, typically 1st. When I tested my '99 hardtop the Gtech numbers were only 2-3% lower than the dyno results.