So, I drove the gas hog the other day (2000 Expedition, 4.6 4wd) and decided to do a little data logging. Upon looking at the plots one thing immediately jumped out at me, at cruising speed the engine loads seem very high.
The Expedition is geared very tall from the factory. At 60mph it turns about 1600rpm with it's 3.55 axle ratio and uber-tall OD. At a steady 50-55mph cruising speed the engine load was hovering right around 60%. Now, this seems nearly ideal except for 2 things:
1) there is virtually no headroom should you encounter something like a hill, a headwind or something else. The difference between 60% and 90+% (and going into fuel sucking open loop) is like 1mm of pedal travel or a 1% grade change. With a vehicle of this size it seems that a little more headroom for changing conditions would produce a better average mpg at the sacrifice of mpg's under all out ideal-conditions (straight flat road with no wind).
2)Only the SLOWEST of slow accelerations keeps the engine load below 80%.
Now, I drive like this most of the time anyways but once again, there is no headroom for things like accelerating uphill or carrying a load. It is insanely easy to get the load spiking over 90%
Do any of you think that there might be better average
mpg's to be had by going to a shorter ratio (say, 4.10 axle)? It seems that it would keep the engine load down a little more and keep it from going into open-loop at the first hint of an elevation change (where it currently drops from 4th gear IMMEDIATELY). That little 4.6 just seems like it needs a little more gear to pull around that heavy truck.
My brother swears up and down his old F150 (same drivetrain) picked up a couple of mpg when he switched to a shorter final drive. I tend to take his opinion with a grain of slat a lot of the time though as I don't trust his methods for calculating mpg.