Tried something new last trip.

Its4mykids wrote:
I always have my tranny in my ’94 F-350, 460, 4.10 gears with OD “turned off” via the button on the gear shift handle when I pull my 12,500 lb fiver. I run it at 63 mph at about 3050 rpm all day long. I found in OD the tranny temp tends to climb well over 200 degrees. Out of OD, it stays around 190. I also installed a vacuum gauge years ago and basically drive by that. I always try to keep the vacuum above 5 Hg. Ran several experiments over the 50k miles and pretty much determined:

1. Throttle angle is very important. In OD, I need way too much throttle angle to maintain speed. Holding the throttle in one spot while in OD, I can turn off OD and gain about 5-7 mph without moving the throttle!
2. Manifold vacuum is important to watch. It is directly proportional to engine load and fuel mileage. In OD, it is rare that my vacuum raises much above 1 – 2 Hg. Most of the time it is at 0. Without OD, it ranges from 7 – 10.
3. WAY too much auto-downshifting when I ran in OD
4. OD is much quieter, but I can handle the added engine noise in the cab.
5. Mileage is pretty much the same either way, with OD being ever so slightly lower. Almost not even noticeable. I find speed/wind is more of a factor in mileage. My fiver is heavy and high profile, so that makes sense.
6. RPM with OD about 2100, without OD about 3050. 2100 is way too low on the torque/hp curve in OD when pulling a huge load (for the 460).
7. 3050 rpm does seem high at times, but then I remember that the 460 is an old engine that ran in vehicles at these rpm’s way before OD was even common in vehicles. It is well maintained, so I let her go.

Bottom line – I have run (and documented) many tests with this truck over 50k miles (probably 80% towing). My conclusion (being an engineer and thinking like an engineer), is to run my truck with OD turned off when towing my fiver. Feel free to disagree.