1/8 bl gearing?

[Patrick]

The 1515 2.5D (1700kv) doesn't rev that high on 4s, so you need to gear up a bit to get your speed, but then too much gearing might coarse more cogging with the mm, not sure.
Are you racing? Not sure what sort of speeds you want, but 35mph is a good area for racing I think. You'll want about a 16t pinion to get ~35mph.
Try BrianG's calculator if you haven't seen it before. http://www.scriptasylum.com/rc_speed/_top_speed.html

I use the 1515 2.5D (1700kv) geared 46/13 in my truggy too, but I use it on 5s to get around 35mph. Seems pretty good how it is, but once I get the handling, and my driving sorted out I'll probably go to a 14t pinion for little bit more top end.