Either the aux tank or a larger main tank.
I took the tiny 26 gallon factory tank out of my Dmax, put in a Transfer Flow 45 gallon tank, and have never looked back. I was telling a buddy the other day that of all the mods I've done, that one is my favorite. Case in point, for the recent Baja 1000 we filled up in El Centro, drove south of San Felipe to BFG Pit 3, and all the way back to San Diego on a single fill up. Kind of funny, my buddy with a stock tank had to fill up in San Felipe on the way home, I considered getting fuel until I realized that I still had more fuel in my tank than he would after filling his rig!
Coan Racing/Dust Junkies Racing, 2012 Baja 1000 Class 1700 Champs
Dust Junkies Racing/Fat City Racing, 2010 Baja 1000 Class 1700 Champs
Fat City Racing/Dust Junkies Racing, 2009 Baja 1000 Class 1700 Champs