Gotta love those big, torquey GMs that come with the gears they need to get good fuel economy too.
I do only a little better in my 1980 LeSabre with 4.1 V6 and 5 speed manual. I blame the carburetor...if it had fuel injection I think I'd be able to squeeze a lot more out of it. My 80 miles/day of driving is less than 40% highway but the rest isn't hectic city stop-and-go, it's town/country roads with reasonably-spaced stop signs and traffic lights.
I bet you would get 28+ in your daughter's Crown Vic, those can get really good numbers.