That is seriously impressive. I had a '19 Hemi with a 3.92 and my average over 72K miles was 14.1 MPG.
Mine was all hand calculated.
I should have made that point before. Besides what rear end you have, it's also important to note whether it is hand calculated or based on what the dashboard computer says.
In tracking my '19 and now my '21 for a combined total of 95K miles, I have found that the dash computer is almost always off. Further, it's almost always high.
In recent fill-ups, I had:
508 miles on tank. Computer said 25.0 MPG. Hand calculation said 22.9.
349 miles on tank. Computer said 26.8 MPG. Hand calculation said 24.1.
506 miles on tank. Computer said 22.6 MPG. Hand calculation said 20.2.
Those are somewhat cherry-picked, out of my last 7 fill-ups, but all but one were higher than what hand calculation said. My '21 is an EcoDiesel, which is how I get that kind of mileage.
It was pretty much the same on my '19 with a Hemi. Lower mileage, but the computer was pretty much always high. Often, by a fair bit.
To be clear, I reset both my B tripmeter and my Fuel Economy gauge every time I fill up. The Trip B and the Fuel Economy gauge always read the same and (obviously) are just based on my driving since the last fill-up - just like my hand calculated #s.
If the #s you reported are from the dash computer, I'm not too surprised. If those are hand calculated, with a 3.92 rear end, then that is really impressive.