if you realize how far you went and what motor you have.... you are getting more then 16mpg and that's city miles that your talking about. for a v6 that is pretty dang good. my 02 grand prix has the 3.1 and I drive like I stole it and I get nearly 280 miles on every full tank which is nearly 16.3mpg. if I take it easy and go long distance taking the freeway, I get nearly 400 miles per full tank. my car has about 224000miles on it and it purrs like a kitten. it is a great family car. gm's 3.1, 3.4, & 3.8 have simple problems such as the head and intake gaskets have an issue of lasting more then 80000-120000miles and need replaced with a new and upgraded gasket, which I have already done my self to my own car and it still runs great like it just rolled off the assembly line
I currently own a 2002 Pontiac Grand Prix GT 2dr 3.8L with 241,000 miles on it & for some reason it's going thru fuel a lot.
I was at 1/4 tank on Monday morning & put in $6 which didn't move the needle a bit.
Tuesday morning I put in $21 in which only gave me 1/4 tank more than previous after driving 270 miles during the week to & from work to home & no extra driving I'm back to 1/4 of tank.
If I can find out why it's doing this will determine whether or not I will keep this car to 300,000 miles that was the goal.