There is some truth to this notion. I commuted 45 miles each day on the highway in a 1981 Honda Civic Sedan. It was rated at 44 MPG and I definitely got that kind of mileage.
It had a 5 speed transmission with 5th gear being so tall that at 65 mph, it was ticking over in the low 2000 rpms. At that point it had very little acceleration, but that was what 4th gear was for. The first four gears had about the same ratios as the 4-speed they offered, with the fifth gear being a kind of highway overdrive gear.
I drove that thing for 8 years to about 200,000 miles and then went back for another one. Much to my chagrin, by 1989, they had retooled the transmission so the 5-speed ratios fit into about the same ratio range as the 4-speed they were offering. What had happened was that gas prices had stabilized and the American public forgot about fuel economy. So the good marketing folks at Honda (and every other mfg) targeted their designs for performance rather than economy. So the point of the 5-speed in 1989 was snappier performance rather than economy.
The question is, could Honda reissue a 45 mpg Civic if the public were buying? I am not so sure they could with the same engine they had in 1981. I believe the CCVC engine was phased out after 1981 because it wasn't very clean and emission standards were going up all the time.
Could Honda or someone else put out a car with a conventional engine that gets 45 mpg? I bet they could, but keep in mind that my 1981 Civic was the cheapest model they had. Their smart money is in putting out very high mileage expensive hybrids and plug-in hybrids.
The Japanese will dod that with really well designed small cars like the Prius which will get very high mileage and be very popular. The Americans will put their hybrid engines in big behemoth vehicles such as the
Ford Escape Hybrid which will pander to the SUV set and still get reasonable mileage. The Escape gets around 30 mph. I think that is a travesty, but that is the free market for you.
I am waiting for the plug-in hybrid version of the Prius or something like it.