There were so many good responses in this thread I couldn't stop and quote em all! Good job guys!
- most trailer tires are speed rated to 65mph. Exceed that for too long and they'll get hot, overheat and blow.....and cause a ton of damage to your rig
- even if you have higher speed rated tires, you have to account for stopping distance, emergency maneuvers, etc.....the higher the speed the less and less time you have to react and stop a combined weight many times in excess of 10k lbs. Simply put you must always be cognizant of the weights and mass behind you. Slow down and enjoy the trip.
- I tow 55-60 and last trip averaged nearly 11mpg with my 6.4/3.73 geared truck, in m5 and tow/haul on. I take my time and stay in the right lane and let the idioits blow by me. Seen a ton of TTs on the side of the road with blowouts...or worse rubber side up....who passed me a few miles prior doing 75-80. Ignorance or stupidity...take your pick...I'm just glad I know better.
- it's a very good idea to upgrade your trailer tires to a higher load range then you need for more reserve capacity. I run load range Ds and have had them on 6 yrs now and they look like new yet. It's a more robust, better constructed tire. I park on wood and keep em covered. I've also been very careful not to curb them, ever. Curb a tire and it'll create a weak spot on the sidewall that could eventually lead to a failure.
As to the OPs question....you'll need to drive slower with any trailer in tow and expect around 10 mpg towing. Remember it's not a speed contest. Take your time and enjoy the tow as I do. Getting there safely is part of the trip and I just love getting out on the highway for a road trip with my rig. Great memories...