I think I posted this elsewhere but, as an experiment over a few recent long runs, I've tried driving at 60-65 mph instead of 70-75 mph.
At 70+ I'd average around 33-35 mpg, whereas at 60 I was getting 40 mpg. Apart from the improved mileage, I noticed that at the slower speed I rarely needed to use the brakes, which meant less accelerating too, instead of the continual speed up, cruise, slow down, repeat etc that came with the higher cruising speed. I also noticed that it didn't seem to affect the overall journey time, with journeys almost exactly matching the google maps estimate.
I found it also a lot more relaxing not to treat the speed limit as a target 🙂