I would be thrilled if I could get 20 highway out of my Hemi Jeep. This will be my 4th Hemi and 20 mpg would be the best I've had yet.
2010 Ram 1500 Crew Cab 2wd 3.92 gears, with MDS: Rated 14/20. Averaged about 15, best highway tank was 19.6 on a long road trip. Most highway tanks were about 16-18. Had that one two years and 50k miles, traded on 2012 below.
2012 Ram 2500 Power Wagon, 4.56 gears: Not rated. Average about 10, best highway mileage is about 13-14, but normally around 11-12 running 70-75 mph. Had this one 15 months and 15k miles.
2008 Charger SRT8, 6.1L Hemi: Rated 13/18. Average about 16, best highway is about 18. Highway could be better but it's pretty hard to drive this car easy. Had this one 2 years and about 20k miles. Trading/Selling for the Jeep.
I know all three of these are a little different animal than the JGC, but just showing that Hemis don't get the best mileage no matter what they are in and that 20+ MPG highway will be an improvement for me.