Just got back from a weekend trip. Here's my stats for highway
mileage on the Magnum RT 5.7L "Hemi":

21.51 mpg and 21.48 mpg on two tanks of 89 octane

This was about 75 percent interstate, running around 80; 15 percent
four-lane divided highway, running 65 or 70; and 10 percent two-lane
running 50, then wide open throttle to 75 when passing, numerous
times.

Trip was about 315 miles each way, which is about one tank each way.

This has astounded my friend who has a Magnum with the larger 6
cylinder. He gets less than 20 mpg on the highway, also running 89
octane. And, of course, it's way better than the Dodge Ram trucks get
with their 5.7L, which is a different engine.