G-tech mph is not measured the same way it will be measured at the creek or at any official drag meet.
The speed reported is literally a terminal speed as calculated at a time when the error in a g-tech is at its greatest ( at the end of the run).
Normally terminal speed of a drag car is the average over a certain distance BEFORE (30 0r 60 feet ?) the finish line and so will be significantly less than as calculated by a g-tech.
In my experience g-techs are a little optimistic, 2 runs are not enough, better to do a whole lot of runs exclude the fastest and slowest and then average the rest.
Also it is a good idea to not slam gears home, esp 1st to second. Using this method it is possible to further improve your times and HP from the g-tech.
about a 30% higher hp figure can be acheived just through smashing 1st to 2nd.