Re: fuel mileage of an Avalanche?
It's in the way the vehicle is tested for CAFE. It is the most idiotic thing I have seen. The vehicle idles, with no load for a preset amount of time (putting this very simply). Emissions are then measured, and fuel economy is calculated from that. That is the official economy listed on the window sticker. This is also the reason that you get about 3-5 mpg less than sticker (reality -vs- statistical). It's all a game.
Big reason between fuel economy in 2001 vs 1980/1990's is the efficiency of the electronics for combustion. Sensors, and computer have gotten faster, and more sophisticated. Aerodynamics play a part, as well as material wieght.
BTW, my 1990, modified 302 gets 17 mpg hwy (75 -80 mph) (15.5 pulling my trailer 65-70 mph), 14 -15 city.