With the overdrive, you can run lower gears than on a 3spd and maintain lower rpm's at highway speeds but your also defeting the purpose of the overdrive gear if you go too low, and being that, depending on your tiresize but I'm shure your not gonna go extremely tall with the tires, a 3.73 would work well, 4.11 would get you out of the hole a little quicker but your going to reduce the effectiveness of the overdrive gear.