Test drove 3.6L with 3.21 & 3.55 rear & 5.7L with 3.21 rear all with RCSB configuration.
Converged on 5.7L for highway fuel economy over the 3.6L.
Drive ~ 35k miles/yr , about 90% highway, no hauling, no towing.
Even at that high mileage, would take ~8 years to recover extra cost of ED at current fuel $.
Believe the only reason to go with ED, is if the truck is used mainly for towing heavy loads like large boat/trailers or travel trailers on highway.
If the truck is used mainly for city, would opt for 3.6L with 3.55 rear ratio. Would not consider the 3.6L with 3.21, even though the 845 Trans is programmed to shift better with the 3.21 rear ratio.
Got the 5.7l RCSB two weeks ago and test drove 50 miles continuously on flat ground at 54 mph with cruise control engaged and averaged 32.3 mpg,based on onboard fuel economy calculator.
90% of the 50 miles, the 5.7L was firing on four cylinders in ECO mode, rather than 8 cylinders.