I have seen a dramatic loss of mpg and power on my 5.4l 1999 F250 S/D 4x4 in recent years. It has the 4.10 gear axle. It still does OK, but not like it did 10 years ago. I've gone from 15 mpg to 11.2 mpg. Towing or not. Current miles are about 225K. It doesn't smoke but maybe time for a major engine rebuild.??? Is there a power tuning device that will improve the mpg and power? Thanks, Huckster