That’s true to point davpal. But a big difference if we are discussing automobiles or tractors, most of the fuel mileage increases in today’s automobiles come from lighter weight, cleaner aerodynamics, better tires, more efficient power transmission (less parasitic losses).  I read a engineering article not too many weeks back that just discussed the gas engine efficiencies between today’s auto’s and those of 30-40 years ago.

so in their research modern timing control added about a 2% in fuel efficiency, fuel injection added about a 1% increase in fuel efficiency, and turbo charging added about a 6-8% in fuel efficiency(debatable according to what I’ve experienced)

so I guess with modern emission requirements some of these gains are erased, so pretty much in the same spot we were 30-40 years ago.