Just completed 4,000 miles...avg of 10mpg

To put it in perspective, if you get 14 mpg at 60 mph in dead flat no wind conditions like I do, you should get 12.8 mpg at 65 mph, and pretty much the same 12.8 mph at 60 into a 5 mph headwind. That's how significant the drag effect is. 

I wish I could tow at 60, but going that much slower than the rest of the traffic makes me uncomfortable. Even at 65 in a 70 means most of the other traffic is going 75 and 10 under has an impact on the other traffic as they try and get around me. It's my motorcycle background that makes me want to be travelling slightly faster than the average traffic speed where I feel the safest along with other threat detection and lane placement. I'm not knocking you guys that can go a bit slower at all, but I just can't do it. 

I tow with my Tacoma. I see a reduction in mileage of a 35% average when towing. Your mileage numbers it within what I see on mine. 

Originally posted by furpod  he found that the "spoiler" actually increased drag.. LOL

 

First a caveat when comparing fuel economy figures. It is very difficult to get an accurate and consistent fuel economy number because very slight changes in wind speed and direction can make a tremendous difference. That is because about 2/3 of the horsepower required towing at 60 is used to overcome drag. Drag goes up with the square of the wind speed and horsepower goes up with the cube of wind speed. Climbing and descending hills makes a big difference as well. Going up and down hills will kill you because you never get all the energy you put in going up when you go back down.

That being said, dead flat, 60 mph, no wind, I get just at 14 mpg towing my 179 with my Highlander. And thats not because the Highlander is a lighter vehicle, the difference in rolling resistance due to that is not very significant and the most of the power goes to overcome drag anyway.

I suspect you are right and are slitting at an unfortunate point on your engines horsepower curve at 65 mph in 4th gear which forces a downshift to 3rd. The additional pumping losses your engine is incurring at the higher rpm are eating into your fuel economy. Try slowing down to 60 or till wherever the truck stays in 4th gear. Pick a no wind day and a flat road, and get your fuel economy number again, see if it goes up to something closer to Glueguy’s and mine. If it does then that was the problem, if not then there might be something else going on with your drivetrain.