Unless RAM made alot of changes with the new Ecodiesel over the previous, I would be surprised if it would give fantastic mileage in the Rebel.
33 inch tires and 3.92 gears certainly won't help the cause.
I had a 2016 Longhorn with the Eco. I tracked every fill up via Fuelly.
Fuelly link
Overall lifetime average: 22mpg
My driving during that time: 95% highway. (110 miles round trip to work daily, cruise set to 78).
The Eco mileage was heavily affected by very small changes. When winter blend hits the northeast, mpg would drop to mid 19.
Add a 12 foot enclosed trailer weighing about 2000 lbs and mpg would drop to 12.
I though about changing to a more aggressive tire tread but, after reading Eco forums, found that a heavier tire (same size) would drop mpg by around 2.
I would be very surprised if the Eco platform in a Rebel would yield anywhere close to the mpg it gives in a small tire with 3.21 gears.
