I followed my friend in his Celebrity once from Ottawa to Toronto and had to hold it under 120, it was quite a noticable improvement in economy compared to avg 130. I've rarely gotten into the 500s on a tank (and that's really pushing it into empty), usually 400s on the hwy and high 300s for city and mixed driving. Re: Ghosst's mileage with his 5.0, I recall on my parent's Buick Roadmaster (the non-LT1 5.7) it would get 800-900 kms on the hwy on its 80L tank, really impressive for a very large car.
