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I've seen most of the fuel economy improvements happen (at least from GM) in the late 00's and later. Much of this seems to come from finally adopting tech like VVT, finer engine control, etc.
Many modern turbo cars now run at stoichiometric almost everywhere on the map under boost. You would never see a pre-2010 vehicle running stoich under boost under any circumstance.
Honda ditched their high flow head from the early '00 engines in favor of a head with a sharp lip before the combustion chamber, trading flow for improved atomization under some conditions.
Offset crankshafts reduced redlines but cut friction and reduced the energy lost from cylinder pressure building before the piston reaches TDC.
Mazda ditched knock sensors entirely and bumped their engines up to 14:1 compression. They use the spark plugs to measure the ionization of the gas post-combustion, along with a powerful computer, to determine what to change before the next time the fuel is injected. Ignition timing is adjusted to the nearest tenth of a degree, and there is VVT on both intake and exhaust cams, as well as variable control of EGR, direct injection timing, etc. I was able to get a 58mpg complete tank cruising at 62mph on aggressive tires in a RWD sports car. Fuelly reports the 1990-1997 models as averaging 25mpg, the 1998-2005 models as averaging 26mpg with a bigger engine and heavier car, 2006-2015 models at 27mpg with 50% more horsepower and bigger/heavier car, and now we have the most recent model year with the Atkinson cycle engine with a 48mpg highway rating, which I see as realistic, and I'm getting a real-world 40mpg city.
Mazda's next generation engine, which uses compression ignition of gasoline, is reported to be 20% more efficient on average.
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