My dad in law offered us his 2003 Buick LeSabre for whatever we could sell a 1991 Subaru Legacy for (which was starting to rust bad), so even though I really didn't like the idea of having such a big car as a family car we opted to do it, also because of safety reasons (we have a two year old).
It has the V6 in it and I think it has the same EPA rating you mentioned.
I inflated the tires to 50 psi and started driving 60 miles and hour with it when taking longer trips. It has been a while since we took a longer trip but the on board trip computer told me we had made an average of just over 40 mpg on a 220 mile trip. This was in the summer, driving slow as mentioned and driving (a safe distance) behind trucks if possible.
My dad in law didn't believe me
In the winter and with mostly city driving however there isn't too muich you can do to make it much mor ufle efficient and I think I end up getting around 22/23 (short trips to grocery store, library, shopping mal, you know the drill).
Hope this helps with your decission.
On a side note; if you look on sites like Craigslist you will see that the actual market value of cars like that are usually well under the Blue Book price, so a realistic market value would probably be more in the $4800 range.