My 98 buick lesabre consistantly got 27-33mpg until its motor blew out at 93k (no mods or hypermiling either) Put in a 2001 buick motor and now its hard to hit 30mpg.
Average was around 29.8mpg but I drove almost all highway on the original 98motor. Too bad there isn't a good way of getting a manual tranny in one of these
Buicks get better mileage natively than most seem to believe, not sure what was done but the cars get good mileage for their size.