I haven’t purchased a new Ram yet; I was referring to my 2014 Jeep Cherokee 4x4 V6 and 2016 Dodge Charger 5.7 Hemi RT. The Jeep is rated for 27 highway, and I average 31 combined, and get 36 highway. My Charger is rated at 25 highway but gets 30 combined and 36 highway. I just use 87 octane E10 fuel...although I am switching to E15 as it becomes more available.
There's a difference between regular passenger vehicles and pickup trucks, which I think was mentioned before (or on another thread on this forum).
The manufacturers only use a few variations to certify their trucks, namely: base trim level, tallest (numerically lowest) gearing, each engine, and 4x2/4x4.
For a RAM 1500, it's most likely that FCA tested a Tradesman in 4x2 and 4x4, with 3.6/5.7/5.7 eTorque, all in regular cab/long bed configuration with 3.21 rear axle ratios. So they test a total of 6 trucks.
When a truck is ordered with 3.55 or 3.92 RAR, loaded up with options (Limited trim level), different cab/bed configurations, the baseline EPA ratings are used.
This leads to wildly different MPGs in the real world. I have been reading forums and checking Fuelly.com for years, and it's pretty rare to see a truck actually attain its EPA rating, much less beat it. Those trucks are typically used on flatter land with lightly-traveled roads.
I had a 2011 F-150 with the 3.5-liter EcoBoost engine, 3.55 RAR, supercab, 6.5' bed, 4x4. It averaged 16.47 MPGs hand-calculated over 36,200 miles.
My 2015 F-150 with the 3.5-liter EcoBoost engine, 3.55 RAR, supercrew, 5.5' bed, 4x4, averaged 16.15 MPGs hand-calculated over 35,800 miles.
For the rest of regular passenger cars, there's usually one, maybe 2 RARs, and they're pretty close numerically. Each engine/trans is certified, and that's about it. Add in the aerodynamic differences between trucks and passenger cars, and it's not surprising to see great variations from the EPA numbers.
I hope this helps.
-John