At least once a year, I have a business trip that takes me to a city that's actually on the same interstate as my town. It's 4 hours on the interstate, and maybe 5 minutes of other driving. To save fuel, I just do 65. But, I have experimented. I don't drive during rush hour, and this is very rural. The city is a city of about 75,000 people, but I live in a town of 4,000. And there's basically nothing in between. Most of the traffic is big trucks. Doing 65 there, 75 back one year I only ended up saving a few minutes. Because I can't do 75 the whole way there. I'm bound to get behind two torque-governed trucks jockeying for position up a hill, dropping down to 50mph. In fact, that's bound to happen several times. Or other situations that slow me down. And the 10mph improvement in speed doesn't actually erase that much time. But it does burn a lot of extra fuel. In my car (every car is different); there's just shy of a 3mpg advantage on the 65mph run. 34mpg at 75, 37mpg at 65 (Speed limit is 70 the entire way, by the way).
The other thing I notice, since; as I said, I do 65 most of the time; is that the same cars pass me over and over. I started paying attention to license plates (pass the time I guess?). So I'd see a little BMW blast by me. And 10 minutes later, see that little BMW stuck in a cluster with big trucks, and I'd catch up with them. A whole would open, and they'd blast off. 15-20 minutes later, there they are again (As I said, not a long in this area, so it's not an uncommon stretch of road to be alongside someone for a couple of hours). The question, to me, is glaringly obvious. What exactly does that speed give you? You burn more fuel and don't make a meaningful improvement to your time!
So yeah, I get you. I'm sure it bugs folks when I putt along in the right lane at 5 below (though, another advantage is, no jockeying for position! Other than passing the occasional 18-wheeler up a steep hill, and then them passing me down a steep descent, I pretty much can just sit in the right lane; no jockeying and passing and getting frustrated by left-lane slowpokes). But it is what it is! They can call me names, I'll be listening to some good music or an interesting conversation on NPR and not playing full-tackle interstate jockeying!