I have had my car for nearly five years now and in that time I never had the urge to check the gas mileage on it.
Having the receipts for the gas used going to Woodward I thought that would be a good occasion to check it, over the 1,004 k. that I drove I averaged 23 mpg. (Wrong - see edit)
Obviously most of it was highway driving and on the Canadian Roads, 401 and 402, bluecoupe and I kept the cars at 10 to 15 kph over the limit. In the States we were maybe 10 mph over. I repeated that on the way home.
I am satisfied with that and promise never to do the calculations again.
I would not like to get in the habit of saving the planet and go easier on the gas. My generation did not care about these things and its too late to start now.
Edit
I slipped up on my conversions, I worked it all out in US gallons and used the conversion factor for Imperial gallons to US gallons instead of its reciprocal.
The mpg should read 27.7 mpg (US.) 33.3 mpg (Imp) or 8.5 liters/100 km. I guess I am doing my bit to save the planet after all.
For comparison our 300 (3.5 liter) gets 10.8 liters /100 Km. according to the onboard computer.

 

My average was 27.2 mpg over a 1300 mile road trip which included my trip from Aurora, Ill to Auburn Hills, Mi. for the WDC and then a run up to the U.P. and then back home to Aurora.

Top speed was 150mph but generally stayed around 80mph for most of the trip.

This was my overall route distance: Driving Directions

 

That's quite the trip for less than a week when its viewed mostly through a camera lens.
I erred on my mpg calculations, they are basically the same as yours. I did not believe my figures when I arrived at them but I could not see my error until I looked at them again today.

 

Back in 2006, Diane and I went on a 6week road trip down through the states; as part of a contest that we held with a group of Xfire owners in Arizona/Nevada/California, we calculated our mileage for the Brantford, Ontario to Cottonwood, Arizona trip that included Dallas, TX, Oklahoma City and Denver, Colorado (the whole purpose of the contest was two-fold....to see who could come closest to estimating my over-all mileage and for my own interest sake, because I had just converted over to full dual exhaust at the time).

Anyways, my average mileage resulted in 29.5 U.S. mpg and I believe the best I got on one tank of gas was approx. 32mpg. I didn't pump up the tires but I did throw it into neutral on the long downhills through the mountains. BTW, our trip did involve a little city driving as well, namely those that I mentioned and a couple of others.

I was very pleased with that and I thought proved that the engine ran much more efficiently with the exhaust conversion. Just my opinion and justification for my conversion...............in my life-time, I should save at least 1 barrel of oil!