Some years ago I had a little 1980 Fiat 127 with quite poor fuel efficiency and a worn out engine. Eventually a conrod bearing seized and as a quickfix I simply ripped out two pistons, disconnected the valves and put it all together again re-using the same gaskets. It then worked hard for another year until I sold it.
Projektblogg - Fiat 127 engine downsizing
(try using Google Translate, the webpage is mainly in swedish)
Those experiments gave me the first important lessons about successful ecodriving. The downsizing gave 10-15% improvement and I soon learned about the fuel enrichment that appears at full throttle and noticed quite an improvement by keeping the throttle at 70-80% of maximum opening. (carburetted engine) I also experimented with staying in 3:rd gear all time instead of continously shifting up and down between 3:rd and 4:th (as you have to do with such a weak engine (about 20 hp maximum I guess). Keeping revs down was definitely a winner! I never tried "Burn & Glide" (pulse & glide, burn & coast) with the little Fiat, mainly because I didn't know about that by then, but with that tiny engine I also needed the engine active all the time to get a decent average speed...
If you run an ICE at a realy low rpm I guess more of the combustion energy will get time to get lost as heat in the cylinder walls and head. At higher revs the losses increase due to pumping and friction, and if you close the throttle to match the actual need for power to keep a steady speed, things will get even worse.
It's all about finding the BSFC sweet-spot and keep the engine there. When that is not possible, turn it off completely! If that's not possible (power steering, power brakes and sensitive engine control computers that go crazy), keep it in idle, out of gear.
I once got a 15-20% improvement in a good friend's cool 400hp BMW coupe, a quite modern car with dual turbos and automatic transmission, using the Burn & Glide method. A nice secondary benefit was to enjoy the acceleration again and again. :-) Since it was an automatic, I put it in neutral after every acceleration cycle, and kept the engine running. After 15-20 minutes the ECU got crazy and started flashing strange error codes on the dashboard. We stopped and let the car rest for a while and everything was back to normal.
Ecodriving worth it's name can be difficult with modern cars, so you can definitely ask if all the electronics are realy a development. I feel we are in some sort of crazy limbo between old simple cars and a true change of paradigm, where everybody rides with hyper efficient automated taxis instead of owning their own cars.