I also have a 2011 F350 SRW CC SB with 355 electric locking axel and 20" wheels and pull a MOntana Mountaineer weighing around 12,000 lbs. While clearly speed is important to mpg, I find that the type of roads you are on make a significant difference. Long distance on freeways, ie never having to stop and start I can get 11.5 or even 12 mpg, crusing at 65mph. Last week we were on Vancouver Island, BC (no freeways, tons of stop lights) and we averaged 11.3.
Overall though very happy with the power and mpg, much better than my V10 F350 (gas) - which generally managed 6-7 mpg when towing.
Chris
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