On roads that have a 65mph speed limit, I tow at about 60-62 mph. That means I don't have to pass anyone. I stay in the right hand lane and allow others drive in the left lane and/or pass me.
Actually, I have found most drivers don't want to be behind a TT and will pass you regardless of your speed anyway.
At about 60mph, I get 8mpg and at 65mph I get about 7mpg. Percentage wise thats a lot (14% increase in mpg by dropping 5 mph). This gives me a little more range so I don'tr have to stop as often for gas.
Steve