Originally Posted by swgachuck
I tried it once but it seemed like it wanted to stay in 4th gear all the time with the converter unlocked...I was thinking it would make the mileage even worse as the fuel mileage readout dropped 3-4 tenths over 20 miles. But, it did keep it from dropping all the way down to third gear and about 3500 rpm when it wasn't in tow/haul mode. Maybe they should have called it the tow/haul/hilly button.
- repeating from other threads / speaking from experience - I've pulled 7,000lbs of trailer plus payload on long, steep grades in 2nd gear running over 4,000 RPM to keep 55-60MPH and my trans temperature tops out about 190F. The engine lets you know it's pushing some power, but coolant temp, trans temp, and oil pressure all stayed happy. I don't know if the 545RFE will lockup in 2nd, but I'd expect that even if it wasn't, I'm so far beyond stall speed that it's effectively locked. If not, I don't think my trans temp would have stayed under control (can anybody answer the lockup question?).
FWIW, O/D is specifically for flat/cruising and fuel economy. Every owner's manual I've seen says to not use it in hilly conditions or under load. "If the truck seems like it's straining, or can't keep speed, you need to take it out of O/D". Also, you weren't dropping mileage because of not using O/D, but because you needed more power for the hills.
Not being a smart-alec, but the relationship and effects of load, RPM, speed. gearing and power-band are not always obvious. Your mileage would have dropped more if you let the engine continue to struggle rather than letting it produce the power it needed.