Geez, you average 21mpg?
I have a 300C (HEMI) that the wife drives. She does a lot of in town driving with it and mileage is usally NEVER above 15mpg on the computer when I go to fill it up. I have had it out on the road a few times and it has seen 24.5, but usually, it runs around 21.5. I do run 87 in it, but doubtful that's the reason. Maybe the 10% ethanol gas? 
It does run like a beotch, though!

Its gotta be that ethanol. Actually, the stuff is lousy as far as gas mileage. Couldn't be the 87 mph.....

80-90% of my driving is non town & lots of that is interstate. I check mileage about once a week but not on the computer - the old fashioned way. The 8 cyl engine has a gas saving feature:
"The multiple displacement system features fast electronic controls, sophisticated control algorithms, and electronic throttle control, moving from eight cylinders to four in 40 milliseconds (0.04 seconds)."
Never notice that but it improves overall mileage.
I watch Barrett Jackson, too. Interesting how the old muscle cars are worth a fortune now.