Not just “one of”, but THE reason. Acceleration (if more than gentle) is a bit less efficient than maintaining speed, but braking is throwing energy away: turning kinetic energy into heat, which is dissipated. (Exception: regen braking like a Prius.)
If you want to get great mileage, avoiding using those nasty pedals!
Bingo. There’s a bit more to it than just driving in the lowest gear without lugging, but that’s a big factor.
I’m confident you’re right. I worked in engine controls for Ford, at their SciLab in Dearborn, back in late 70’s early 80’s. The algorithms for calculating things like fuel and spark advance were called “strategies”, and even way back then (even on 8-bit processors) involved a lot of factors, and a lot of compromises between performance, emissions, and economy. (One of the trickiest parts back then was modeling the fuel layer thickness in the intake manifold.) Once the strategy was established, there were still lots of tables of “calibration” data that were massaged by technicians to suit a given car/engine combination.
I wouldn’t be surprised to find that the ideal gas mileage speed might depend on which technician did the cal for your particular vehicle. (Before cal, cars were barely driveable, despite all the science and math that goes into calculating the tables in the first place.)
No doubt things have come a long way since then, and I wonder if there are many central fuel injection models (which would still have an intake manifold). But I bet they still have technicians who calibrate the tables, and even if not, the optimum speed would depend rather strongly on the control strategy. Of course, it would also depend on transmission shift points and gear ratios, and aerodynamics.
BTW, even back then, the research showed that at highway speeds (55), most cars were more efficient with AC on and windows up. That was before they even got rid of the drip rails, but after they’d curved them inwards to reduce drag, IIRC. I’d expect the benefit to be bigger today, with sleeker cars.