2018 Ram fuel milage questions

my 2014 slt with 30,000 kms has 3.92's and there is basically no difference in mileage between mine and my brother in laws 10,000 kms 2017 big horn with 3.21's. I think theres about 100 rpm difference at about 65 mph between the 2 or so.
Personally i love the truck, good power and driving sensible in the highway I can get high 27/28 mpg (calculated with imperial gallon here in Canada)