For a given speed, 3s will run a lot cooler than 2s, The Slash 2wd offers a wide enough range of gearing to work on 3s or 2s with either of those motors, so I'd start with a pinion that is 2/3rd the size of whatever you like on 2s (or just pick a pinion and spur that gives you 25 or 30mph with whatever combo you want (30 should be fine unless it's really, really, hot where you are, but it sounds like it might be so I'll include both), per http://www.scriptasylum.com/rc_speed/top_speed.html
, if you're running stock tires
2s 4600 25mph: 14/90
2s 4600 30mph: 17/90
3s 4600 25mph: 9/90 (I don't know if a 9t pinion will fit)
3s 4600 30mph: 11/90
2s 3800 25mph: 17/90
2s 3800 30mph: 20/90
3s 3800 25mph: 11/90
3s 3800 30mph: 14/90
I picked the pinion that got you closest to the speeds listed, you'd probably be fine being off a tooth either way. I hope this helps as a starting point!
Sheesh I made this post a whole month ago and just now getting updates or something on it, I thought I was doing something wrong but an update on my truck.
-So right now I have a 3800kv motor in the truck with an Integy motor cooling fan and heat sink on it and the sidewinder sv3 ESC with a cooling fan
-I'm currently running 14t 48P pinion gear and 90t 48P Spur gear (both Metal or Aluminum), I do have a 16t Pinion and 86t spur gear for more options but I've read a lot on forums that gearing for highspeed will raise the temps.
-During the day it will get 120+ degrees around this time of the year and we're not even in August yet *Palms face* so you can Imagine how my days are smh. So I try to only run my Slash during the night time or when the weather wants to act like its cooling off
-I have 3 2s 5000mah 30c batteries and 3 3s 5200mah 50c batteries I really only like running 3s with my 3800 because its fast when I put the 2s in it I'm just like Ehh okay not really fun
-Ive upgraded my shocks to the Big bore shocks and I'm going to put 60wt in the rear and probably keep 50wt in the front or when my 40wt oil comes in i'll throw that in the front
-I am not running stock wheels as they are some trash out here in the desert and the truck wants to be doing donuts in the sand without me even keeping my hand on the turning knob lol so I have the Proline Badlands (rear) and Proline Trenchers (front) on the track that we have those tires give me awesome freakin Traction so much that the truck flips while cornering, at first when I had the stocks on the rear I had no traction on that track. Just yesterday my order of Proline Badlands 2.8 came in now I'm just waiting on the Trenchers 2.8 to come in I'm going to convert it into a monster tuck
-I'm currently looking into putting a 2200kv motor in the truck HorizonHobby has this (MAMBA X 25.2V WP ESC 1406-2200Kv Sensored Combo), I want to do more research first like find out different aftermarket parts I would need to handle all that power and if I do get this combo I am going to get 4s batteries. I love this hobby and would like to see different systems the truck can handle
That's pretty much the upgrades I have on it now, a couple other aluminum parts but that's it I do not regret buying this truck it is so much fun