I didn't want to hijack the 4 cyl thread, so I thought I'd start a new one. This info was compiled on a round trip from Rogers, Ar. to Daytona Beach, FL., from December 26th to January 1st.
The truck is a 2009, DC, TRD Sport, V-6, Auto, 7,000 miles. Spring and trans TSB's have been done.
The trailer is a 12ft FeatherLite, 6' 2" high, 6' 8" wide. It's a V-nose, so it's about 14' 6" long down the center. Loaded with two enduro roadracing karts, spare motors, 35 gallons racing fuel, 3 tool boxes, and many spare parts, it weighs 4,600lb. The trailer does have brakes, and I use an Activator III Brake Controller, installed as shown on Maverick's Towing Bible thread.
Data was gathered from my Scangage II, but verified with a calculator.
Average mileage was 10.6 mpg for the entire trip. The low was one tank at 7.8 mpg (from Valdosta, Ga. to Atlanta, Ga., with a 20 mph headwind.) The high was from Memphis, Tn. to Clarksville, Ar. at 13.4 mpg.
I drove the truck in "D" 90% of the time with the cruise control off so I could control the shifting. At 73 mph on flat road (2,100rpm), the truck would stay in 5th with the "Instant MPG" on the ScanGage showing 10.8-11.2 mpg.
On gentle grades the torque converter would unlock and the rpms would rise to about 2,500rpm and the instant mileage would show 7.8-8.4 mpg, maintaining about 70 mph.
When I would come to steeper grades, I would manually downshift to 4th. Rpms would rise to about 3,100rpm and Instant Mileage would show about 5.4-6.5, still maintaining about 70mph.
I used to tow a similar trailer with a 2007 Expedition. I could put in OD, set the cruise control on 72mph, and forget it. Mileage was always about 10-12mpg. It was a good rig, but I couldn't handle the 13mpg around town, plus I needed a truck. I tow to about six races per year, so I spend a lot more time not towing, than towing.
I plan to install a TRD Supercharger in the next few weeks as I am told the towing mileage will improve since more torque will allow lower throttle settings to maintain the same speed.