Gearing on an E-Maxx with a 7018 and _ Motor (also picking a motor)

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I don't understand how you need to gear a setup to prevent thermaling. Is their a formula I can use? I found one recommendation to keep the E-Maxx at around 18:1 gearing. Is that true? Why doesn't that apply for the HV-Maxx, but it does for the 7018?

Also, from what I can tell I could use anywhere from a 9L up to a 10XL and still be OK based soley on amperage draw. I plan to use 12 and 14 cell packs. Yet, I know that I could overgear the truck and fry the ESC.

I guess I don't understand why I couldn't pick say a 12 L and then gear the truck to get the speed I want... does it have to do with starting out torque?

Any motor recommendations or am I fighting a lost battle with the 7018?