[Patrick]

Either the 1515 2.5D (1700kv) or the 1515 1.5y (1500kv)
Assume your using the e-maxx trans with the 51t spur? And stock revo diffs?
2.5D 51/16 should give around 40 mph.
2.5D 51/18 or 19 should give around 45 mph.
*51/16 will not fit when using the stock e-maxx motor plate and a finned neu. The motor hits the trans before the gears mesh properly, there are ways around it, but they're not that pretty.
1.5y 51/18 or 19 should give around 40 mph
1.5y 51/21 should give around 45 mph.
*With 51/21 you may need to extend the slot on the e-maxx motor plate a mm or two, because the motor will be so far from the trans.

I race the 2.5D on 5s geared for only 35 mph, but it barely gets warm. I've done a bit of high speed bashing with it geared 51/20 on 5s and 6s in a maxx. On 5s it's really fast and powerful with that gearing, on 6s it was actually getting kind off scary even in a large grass sports field....I love that motor, but I think the 1.5y may be better suited if you only plan on running 6s, mainly because the 2.5D has more power than you need for what you want. Temps weren't an issue. Both will work well though if you gear to suit want you want.