I use cruise control aggressively, if i have it. That is, i use if even if it only marginally makes sense. That's because it gives me 4% to 5% fuel economy at the same speed. And, it generally reduces my workload.
I consistently get 17% better fuel economy for going 62 MPH rather than 70 MPH. Why 70? Michigan's highest speed limit is 70. I try very hard to not speed. Why 62 MPH? My speedometer also shows KPH, so there's a handy reference - 100 KPH. Naturally, Edmunds gets a higher percent improvement. Often overlooked is that driving slower is safer. We tend to increase our risk to some threshold. I try to minimize my risk unless there's some corresponding benefit. Usually there isn't.
Drafting: I've tried drafting manually. I found that drafting a truck fairly got me worse fuel economy than driving at the same speed. And, driving at the same speed with cruise control is better yet. Drafting is risky, and pointless, so i don't do it. This test was done on a cross country trip, covering about 400 miles each. Wind and terrain were similar. My guess is this. Constant speed changes, through brakes or engine drag, kill fuel economy. Engine drag is the same as the brakes.
Roof luggage. I lost between 5% and 10% for putting roof luggage on my car. Cruise control was used at the same speed on the same course for thousands of miles. My luggage carrier at least looks fairly aerodynamically efficient. The way i look at it is that you put roof luggage up there if you need it. I'd love to be able to drag a trailer. It would be better than driving a pickup and paying the cost all the time.
A/C. I've not done this test. I'm not into noise, so i don't drive with the windows open on the highway. I've read articles about just this topic. The answer seems to depend on the details; such has speed, the kind of car, and so on. It's not a huge difference anyway.
Tire pressure. I underinflated the tires on my car. It did not change my fuel economy in any measureable way. I overinflated my tires, and again, no change. But i view it as a safety issue. For at least one of my historic cars over inflation leads to over steering.
Manual transmissions. I've heard otherwise, but my tests show that manual transmissions save you at least 5% in fuel economy over automatics. I'm not a gear head who likes to shift gears. What i'd like is a series hybrid. That is, and electric transmission. With no differential, we'd get an additional 7% to 15%. With no engine drag, we might get another 20%. There shouldn't have to be any gears, so there's smooth acceleration always. It should last longer than hydromechanical automatic transmissions. It should have redundancy that will get you home even if a wheel can't be powered. It should be cheaper to replace if it does fail.
My conclusions match too. The main issue is psychological. I used to have an hour commute to work. Driving faster could save me, at most, about five minutes. But since there were traffic slowdowns, it was impossible to measure five minutes. Psychologically, it's difficult to let everyone pass you. This is the biggest hurdle. But i can use the cruise control much more often at 62 MPH than at even 65 MPH. Everyone else has to pass me. I have the right of way. It's worth something.