I can offer a comparison of the affects of the transmissions being discussed based on my previous two 1500 Big Horns, both Quad Cab, two-wheel drive, with 3.92 axle ratio and 20 inch tires.
My 2010 had the six speed and finished a lifetime average of 17.67 MPG at the end of four years, 104,000 miles. I had 20 tank fills that averaged 20.0 MPG and over (highest recorded was 24.01).
My 2014 with the 8-speed finished with a life time average of 18.64 MPG at the end of four years and 94,000 miles. I had 52 tank fills that averaged 20.0 MPG and over (highest recorded was 25.81).
This represents only one person's driving habit, style and topography. Your mileage may vary.
Regards,
Dusty
2019 Ram 1500 Billet Silver Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33-gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 3 June 2018. Now at 79608 miles