I drove a 2014 Double-Cab 4.3 last year. I pulled several trailers with the vehicle (including those that may have exceeded the capacity). I logged close to 15k miles on the truck in the 4 months.
I have been driving a 2014 Double-Cab 5.3 3.08 for the last couple weeks. It has been mostly tow duty with the same trailers, with some non-towing.
I could not tell the difference.
So far non-towing, the V6 had superior mileage. The cut-off to V4 left the 4.3 having more power than the 5.3. I will now be commuting in the 5.3 for the next week, and see if I can average 20 mpg. I was in the 21-22 last year with the 4.3.
