I get 10 mpg with no trailer with my V-10. 6-8 mpg's with a trailer depending on size/weight (295's & 4.56 gears). Its embarrassing and probably a reason to sell my QuadVan.
I have resorted to towing with a DuraMax. Much, much more efficient fuel wise & power wise.
I tow a Vette or an old bathtub.
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Two wheels or four, its all good. :-)
'10 E-350 EB V-10 QuadVan 4x4, SMB penthouse top, Van Specialties interior