99% of the time 'od', however I just pulled my Sisters Pontiac Montanna on an Uhaul auto transporter(the tandom axle trailer, not the tow dolly) to Toronto(5 hours each way) last night. At first I had it in od with the cruise set to 110, but the truck was hunting for gears all the time, so I dropped it to 3, the rpms averaged around 2300, tps average was 18%, and the engine load average was 24%, didn't drop another gear until big hills came around. As for gas mileage? Didn't cost me anymore than a typical trip to TO, just shy of a tank of gas each way. Blew my mind that I could pull 5000+ pounds running at a higher rpm and not burn more gas. Wouldn't have believed it, if I just didn't do it. I think that dropping it to 3 helped.