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.