I have the sense that the 3.21 gearing makes a big difference in fuel economy. I have the 3.92, and intially it seemed exciting to have the better accelerating gearing. However, before buying I also trusted that I really would see 17 city 22 highway. HA! My combined says 16, but when I calculate economy at fillup, it's more like 15, 15.5. So now I wish I had the 3.21.
My question to you is, has your fuel economy been consistent, or has it changed over time? You say 566 mi is the best ever since you had it. My Dodge Dakota, now 20 years old with 198,000 miles, didn't vary much in its history. Pure interstate (long trips) offered the best. So I don't expect fuel economy to get better.