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
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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.
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