Lots of factors to consider here:
I'm a car guy that has been maintaining, modifying, and repairing just about everything on my cars since I was a teenager (30 years). My most recent cars that serve as a reference (all bought new): 2001 Pontiac GP (3.1) which I still have, 2004 Impala LS (3.8L), 2005 Chrysler 300 Touring (3.5), 2008 Chrysler 300 Limited (3.5L) and my dad's 2001 Buick Lesabre (3.8).
My 2008 Chrysler 300 Limited gets about 24 MPG per gallon on the highway. I can go about 330-350 on a tank full of gas withy the 18 gallon tank in my car. The car computer shows that overall in the last 10,000 miles I have averaged 20 MPG, which is all driving. I pretty much drive between 70-80 MPH on the highway, with 80 (2,500 RPM) being more often when possible. I have the 4 speed auto, not the 5 speed. My 2005 300 averaged about 27 MPG on the highways but with the skinny 205/65/17 tires. Skinny tires increase fuel mileage. My 2008 has 245/55/18 ZR BFG KDWS's in place of the 225/60/18's (with these tires the car will get about 26 MPG on the highway). I also have a K&N airfilter in the stock housing-no other modifications. The biggest obstacle to the mileage on these cars is the weight of the vehicle-IT's heavy-about 3,800 for the 3.5's, over 4,000 with the Hemi.
My 2001 Pontiac GP with the 3.1 weighs about 3,400 lbs with about 190 HP (some modifications, K&N, magnaflow duals, u bend delete) and it will get 30 MPG on the highway and around 400 miles on a 16 gallon tank. 2,300 RPM @ 80 mph with the 4 speed auto. 2004 Impala LS (3.8, 205 HP) weighing about 3,500 lbs turning 2,000 rpm @80 mph would get around 26-27 mpg.
Four basic factors effect fuel mileage:
1. HP of the engine and number of cylinders (also size of the engine 3.1 versus 3.8)
2. Weight of the vehicle in comparison to HP
3. RPM's at steady MPH (transmission and rear end gear ratio)
4. Width of tires (wider, more rolling resistance)
Hope that helps!
