Just as there appears to be such a huge difference on what owners are claiming for mileage as compared to what I and the people I know with the '08 get (generally lousy), there are probably substantial changes in how each engine will respond the to the zillions of sensory data inputs the PCM receives. I hear the program itself is a monster - millions of lines of code. Add in altitude, weight, tires, road surfaces, driving patterns, weather and so on, I'd expect input/outputs to be variable, not static. Regen times are triggered by a multitude of inputs, so the response will vary, no doubt. Most regeneration processes are terminated by a prime target input, more often than not it's a preset temperature. If driving conditions are so varied, then the time to reach the reset temperatures will be all over the map, dramatically changing the duration of the regen cycle. It's the last few degrees of the cycle that increase at the slowest rate.
On a positive note: I hear that the big 3 are lobbying hard where they can to have the guidelines changed. It's their response to lousy mileage complaints and therefore must be the basis for their argument that I'd guess would be what some of us have been asking all along - if you burn MORE fuel how can you possibly have FEWER emissions? I was told by a dealer who I trust (there actually are such people!) that by next year there should be a drastically different update available for the 6.4 that will specifically address the poor mileage issue. Hopefully it will directly deal with the necessity of, or better the lack of, this awful DPF and all the catalytic garbage with it. For an engine that's designed to be capable of 50 lbs of boost, it NEEDS it. Mine runs, on dry flat pavement, empty truck, between 12 and 16 lbs and that's just to barely maintain highway speed! At 16 lbs of boost, my 6.0 liter would be approaching tire speed rating and still accelerating! I would think that Ford would be leading the pack with the builders of the 6.7 Cummins right there beside them especially considering the takeover of the number 2 position in North America for vehicle sales by Toyota, punting Ford down to #3. Anybody driven an '08 Tundra 4X4? Wow. Beats any half ton I can think of hands down, and power to burn and still hand them all their collective a**** in the fuel consumption department, reasonably same sized engines compared. If they ever make a 3/4 ton with a diesel, look out big 3. If the economic state of the US is gauged by looking at the state of General Motors, then dark days would be ahead.