I had the 3.92s in my 2010 (6-speed) and my 2014 (8-speed). At that time I had a requirement to tow close to the 10,450 pound limit. Since that requirement disappeared, I ordered my '19 with the 3.21 differential and I now tow just under 7000 lbs. The 3.21s handle that just fine and my day-to-day average gas mileage has increased about 1.2 miles per gallon (lifetime-to-lifetime average).
Kentthelawnguy feels his with 3.21s is actually more responsive, and I have to agree. There's no doubt that 3.92s will likely launch better (if you can control wheel spin), especially with a 1st gear ratio of 5.000, but I find with the 3.21s 3rd and 4th gears seem to pull better at road speeds probably due to the 5.7s torque rpm band.
Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045561 miles.