[Frank Lee]

I think several of OP's numbers and expectations are skewed.

I doubt the 300 returns about 30 mpg @ 75 mph. Fuel economy.gov says 31 highway which may be where OP got that number BUT EPA highway is NOT 75 mph cruise; it is a 10.6 mile dyno run with average speed of 48 mph and never exceeds 60 mph. Or he has a dashboard economy gauge that says 30 @ 75 but... that is likely a temporary value at optimal conditions like a slight downhill grade and/or tailwind. I wouldn't put too much emphasis on that gauge reading; I'd use hand-calculated tank-to-tank values for a reality check.

Furthermore I doubt the drivetrain is only operating at 85% efficiency at cruise speed, nor the engine at only the default value of 22%. Bumping the engine efficiency to 25% and the drivetrain efficiency to 90% results in estimated 31 mpg @ 55 mph and 20 mpg @ 75 mph which seems reasonable to me (although I have no personal experience with the Chrysler 300).

Being a little more generous with Crr- to .010- gives 33 mpg @ 55 and 22 mpg @75; .010 Crr is entirely (PUN!) plausible and the predicted results seem to be as well.

There are only seven 300 owners on fuelecon.gov sharing their mpgs but it looks like 26 mpg is a reasonable highway mpg expectation. We don't know how fast they drive though. One user reports 31 highway and if he cruises at 55 mph that fits nicely with the calculator's estimates.