I recently took a trip about 90 miles due east this past weekend, and got fairly respectable fuel economy with an average speed of about 63 MPH. Most of that was about 75 MPH, moving with traffic. My MPGuino says I got 27.8 MPG, which is 3 MPG above the EPA highway rating for the 2005 Dodge Magnum with the 3.5L V6 engine.
Trip to Rock Island
Return trip (had to fill up at MP 284 on I-80)
(for the MPGuino displays above, the upper left and lower left values are time values in HHMMSS format. The upper left value is engine runtime, and the lower left value is time the vehicle spent in motion).
My Magnum currently has a 2.87 rear end, and I am considering going to a 2.65 from a newer Chrysler LX platform car. I figure it should net me roughly a 4% increase in fuel economy, and apparently I can find one for $90 plus shipping on eBay.