My 2015 came with a tank of E85 and was garbage on that fuel. What are you putting in it? My lifetime average on the 2015 with 75/25 highway/city is in the 18 range mostly on E10/E15 87 octane.
Did you have the BCM reprogrammed for the wheels? If not, you're likely seeing a big difference, and that difference is magnified the faster you go. When I got the 2015 and put LT 20" wheels on it, the dealer claimed they had reprogrammed the BCM, but a GPS speedometer app confirmed they had not. I went back and they took care of it.
If your speedometer is not properly calibrated, take your rig in and get that fixed. It's worth the money. You're creating a problem down the road when you go to sell and the true mileage is off.