Cornelius, have you placed an order yet? My 2015 was a Sport RCSB with Ramboxes and 3.92’s. It was a blast to drive and I was getting hand calculated mileage of 19.5 to 21.5 mpg during the warm time of the year with stock GY SRA Tires. 80% highway driving mostly speed limit or 5 above. When it turned past fall I lost mileage. 2 factors: winter blend gas & installed Cooper ATW tires.
I’d say around 1 1/2 mpg. That truck I owned 20 months and had 53,000 miles when I traded it last year.
Then I purchased a 2016 Longhorn CC, 6.4’ Rambox, air suspension with 3.21 gears. This being a 4 door, and heavier, the best I could do was 20.5 mpg. Mostly getting 18 to 20mpg. And again with the winter gas & my Cooper tires on, it was 17.5 to 18.5. Again mostly highway driving.
I just traded the 2016 (39,000 in 12 months)in on a 2018 Limited. Again, CC, Air Suspension and 6.4’ Rambox bed. This truck has 3.92’s.
Haven’t had a mileage check yet, but the EVIC is only showing 17mpg. Just started a new work location, so at 170 miles round trip a day, I’ll soon have some hard data on mileage.
Mainly I’d say, the 3.92’s are more fun, and it’s what I wanted, but I’m expecting to be buying more gas for sure. With the 3.21’s, the truck ran fine and never felt like it was a dog. You’d be happy with either one.
Can’t really compare with your 2008 because of the difference in the transmissions. The 8 speed works well with either gear set.
Good Luck in whatever you choose!