I consistantly got 18 on the highway out of my old 97, running 75 MPH. Only problem I had was undersized brakes on the front. It ate them up, easily heated up and warped rotors.
I would not go with the newer generation because I do not like the drivetrain. The 5.3 with 3:73's just do not get the mileage. My 04 with this 5.3 3:73's got about 14MPG on the highway running 75 MPH.
That is totally backwards from my experience. Having also owned pickup with both drive combos, the 5.3L always gets better mileage than the 5.7L with everything else being equal.
-Coolant leaks from the lower intake gaskets. GM used plastic intake gaskets impregnated with rubber around passages. With enough heat cycles the plastic breaks and starts leaking coolant out side and sometimes INSIDE the engine, contaminating the oil. If that's the case, I recommend the Fel-Pro gaskets. They are metal with rubber, and allot sturdier.
Love my 99 Yukon 5.7, only has 116K, only issues was fuel pump at approx 95K. Average mpg is 14-16, no tranny issues, chg all fluids regularly and it's been on synthetic motor oil since 75K, nice, comfy ride and the posi rear end is great.
