So 20% of his fuel was just dumping out the tail pipe un-burned? Either he changed something else like mabey added a chip or taller gearing or... mabey he doesn't want anyone to know he got suckered so he is throwing out some dreamed up number The only real world experiance I have had proved that the physics was right, takes more energy to run them than what you get back from them.

Nope. I've known him since 1978. He isn't a BS'er. It was a bone stock Cummins, and gearing.

You can believe it if you want.

It all in the cruise rpm and the locking up of the converter all in the most Efficent spot. I so believe hot rod. This is not the first time I heard this and two very good and long time freinds told me the same thing. The truck simply just hot better mileage on the hwy. towing something heavy.

I think it may having something to do with the converter and the rpm range which the engine is running at while towing forcing the converter to work better/harder.