I am not worried as much with expense as with reliability. Do you think I will be ripping diffs apart constantly on 6s with the 1515 1.5y?

All I really want is a solid 40 mph. Your help and advice is much appreciated.

Stock diffs would be lucky to last 5 minutes, upgraded diffs would last maybe 10 minutes. The RCM drop-in 1:8 conversion is the only option for bullet proof diffs really- not cheap, but cheaper in the long run since it saves replacing broken diffs every run....

I don't think you've go through diffs that fast really...

Ditto. I'm still using the totally stock diffs in my Revo and they are still fine (for now). As long as you don't totally mash the throttle all the time, they seem to hold up OK.

Oh oh, I always mash the throttle a lot on everything :lol::rofl::whistle:

Well, I think he should go with 6s lipo setup. And I agree with the 1515/1.5Y choice, that should be perfect amount of power and feel in my opinion... I am thinking of getting that motor for myself for my 7s2p setup.

Personally I would get the 1515 1.5Y for using 6s only.
You'll have to use 20t pinions and bigger with the 51t spur to get speeds over 40mph as it is with the e-maxx trans and 1/8th diffs. Using the 3D and even using a 25t pinion you'll only be able to gear for ~43mph.
The big pinions are a bit annoying to use, because the pinion is so big it covers the screws that hold the motor on, making it harder to set the mesh properly.

I am considering a setup like mkrusedc's and running 4s since that works well for him and seems to be more in line with the speeds I need. After seriously considering everything, I think 35-40 is a better top speed range for me. As long as the setup is reliable.

Mine has been very reliable. If you have seen my videos :oops: those are stock diffs with 3.3 shafts and a stock slipper. I did not even put a flat spot on the motor shaft for the pinion screw. I don't know why it is holding together so well, maybe I am just lucky, but I have about a dozen runs of heavy bashing with that setup.

:mdr: Mark