I just had a long boring drive back from K-Zoo, so I decided to do a little messing around with different speeds to see how much difference it made with MPG. My truck is a 2011 crew cab ecoboost f150 4x4 with 305/50/20 nitto terra grapplers with 4.10 gears.
For all the different speeds, I set the cruise control, hit the mpg reset and drove for 25 minutes without touching the gas or brake. Luckily, there was little to no traffic on 94, so I was able to keep it consistent.
65 mph- 19.8 mpg
70 mph- 17.6 mpg
75 mph- 15.8 mpg
80 mph- 14.1 mpg
Now this obviously isn't perfect due to hills, wind, turns, etc...but it gives a decent representation since I was heading in the same general direction, and I-94 is fairly flat between K-zoo and Ann Arbor.
For the first 10-15 minutes after I hit the reset button for each speed, the mpg's would jump around a lot depending on the road conditions...but it eventually stabilized. After 25 minutes, I wasn't seeing any more changes...so I would switch up the speed and start over.
I never paid much attention to the MPG, but after seeing how much difference a few mph makes, I may have to slow down a bit...I usually drive 75-80, but with that drastic of a drop in economy, I may have to cutit back to 65-70. It makes sense though, the small displacement turbo engine needs to make quite a bit of boost to keep the speeds that high, especially with my heavy tires with a lot of rolling resistance and aerodynamics of a brick....so it drinks the fuel down fast.
/useless information that kept me amused on my 2.5 hour drive.