Hi Lonestar, you're getting over my head when you start talking changing rear end gear ratios. But my first diesel was an '05 F-250 PSD. My Airstream weighs about 7,000 LBS loaded and my truck had the 3.73 rear end and was a two wheel drive, extended cab.
The transmission in the PSD actually has six gears. You only use five of them when in either regular or tow/haul mode. When you switch to the tow/haul mode, you use four of the same gears that you use in regular mode and switch out one of the mid-range gears for a towing mode gear to maximize your power band. I forget which one. Plus you change your shift points as stated before. My '05 shifted into the final gear (overdrive as you say) in the tow/haul mode at around 65-70 MPH. Again, as was said, depending on the load on the engine such as hills, acceleration, etc. I always used the cruise control on the interstate and got 12.6 on average including having the golf cart in the bed of the truck (increased wind resistance). This was setting the cruise on, dare I say it, 75 MPH. At this speed, BTW, my tach read the same as it did at 55 MPH in tow/haul mode (same tach, different gear).
In December I traded trucks because the seats were uncomfortable and really didn't fit my back and had begun to irritate an old lower back injury to the point that I was having to undergo physical therapy. The '08s have been redesigned from the ground up including the suspension, drive-train, interior including seats, and the exterior styling.
The styling was a non-issue for me, but the ride was what was important to me. The seats in the new design fit my back perfectly, the ride is more like that of a 1/2 ton and the only time I really get the firmness of the 3/4 ton suspension is with the golf cart in the bed and the trailer in tow.
The bad part is the fuel economy. I started getting 9.6 MPG's towing and after 16K miles, I don't get much better with or without the golf cart. Part of that is the new ULSD diesel fuel. Part of that is the "afterburner" (diesel particulate filter in the exhaust system which uses diesel to burn off the particulates) and catalytic converter that gives the emissions equal to a gasoline engine. When I got my first oil change I asked the dealer to check the programming and see if the mechanic could get me any better mileage and told him my driving habits. When I picked the truck up, I was told that there had been a service bulletin issued and they had updated the program. This got me about an additional MPG on the daily commute and about one when towing. Disappointing, but an improvement just the same.
I would suspect that you are either towing too slow to reach a speed to shift into overdrive in the tow/haul mode or you might need to ask your Ford dealer's service department to see if there has been a service bulletin issued to reprogram your computer control system.
Either way, always use the tow/haul mode when towing. It is there to protect your transmission and with the heavy load you tow it isn't worth stressing your drive-train just to save a few...ok quiet a few...pennies on fuel.
. So what I was wanting to know is how is every body else doing. What truck are you pulling with, how much weight are you pulling, and will it hold overdrive. Oh yeah whats your mpg while towing ??? Thanks in advance.
. I cant get into the final gear thanks for the reply.
