Revo conversion

So I'm finally converting my revo with my 1515 3D and have a few questions before I finish.I'll mainly run 5s for now with a wide ratio tranny locked in second.what kind of gearing would I need for 40mph? I realize the 1515 3D I'm using is capable of much more but I'm looking for longevity of the the drivetrain and general bashing. And when my skills get better with a truck this fast I'll jump up to 6s and re gear appropriately.thanks--josh