I took a measure of a two week span on my car. I drive 88 miles a day to work and back for 9 days, plus about 20 for various stuff for kids and wife. Mixed driving about 4/5 of it is hwy miles and the "city" miles are actually small towns along the way with a speed zone and a red light or two.
avg speed 41.6mph
avg per gal. 25.2 by car and 25.14 by my calc
total miles 816 miles
I ran Shell 93 octane w/10% Ethanol, since i work at one of their refineries i might as well invest in myself.
I used automatic, not tapshift, and yes i did show my butt a few times to some trucks that would try to out run me to the lane cut off.
I also use cruise control to keep from looking down and seeing the speedo at 80 or 90.
I did a short check last night while we were in BR and was getting 15mph over a 28 mile trip while looking for baseball stuff for my 7yr old.
My city driving in this car is about the mpg i was getting out of my truck on the hwy. So i'm rather pleased at my numbers.