While I will say this is true
While I will say this is true for most cars, especially cars with 3 and 4 speed autos (God help you if you're still driving a 2 speed Powerglide around) and even my '04 Cobra that in 6th gear I'm @ (I think) 2200 rpm's @ 65 mph. The 2013 GT500 has such a high rear, 3.31 that you cant even put it in 6th gear until over 70 mph...so keeping it in 5th around 2k rpms @55 mph uses more gas than putting it in 6th gear @ 80mph while "just" seeing 1500 rpm's. Can't argue with me, I already tested it, and the difference is +4 mpg in 6th gear @80 mph @1500 rpms.