I have a '00 LS6 with 145K miles. Just recently put new O2 sensors and cleaned & re-oiled the air filter, have brand new front tires and 2-year old rear ones. On long trips I get 21-22 mpg, and locally, which is still mostly highway, I get 17-18 mpg. My friends' trucks give them better mileage! I mean, is normal for LS6? What do your cars get?