Heh heh... now that I've peaked interest... I'm not talking about some magic carb or anything.
In the 1970s a fellow built a serial hybrid car that was able to maintain 45MPH all day long, while getting 70MPG. He used a jet engine starter and a 1950s generator setup to run this 36V system. Mother Earth News duplicated the project, only using a larger car, larger generator and higher voltage, and they were able to get better mileage.
Now picture someone like Ford or GM building one, and taking it seriously. Picture a purpose-built electric motor driven by a purpose-built generator, running 360V. No battery pack for extended range driving, just a small battery pack to act as a load balancer. Picture the genset using a direct injection turbodiesel engine, optimized to run at a specific RPM with the ratio between the generator and engine matched so that the diesel's most efficient engine RPM matches the generator's most efficient RPM. GM actually came close with the Volt, but they saddled it with that useless 1000lb battery pack. Without that battery pack they could size the engine, generator and motor to move a lighter car, and would likely hit the 100MPG mark.
If a man in his garage could build a car that gets 70MPG with a top speed of 45MPH using cobbled together 1950s technology, then an OEM with a multi-million dollar budget could certainly build a car that would operate with the same performance metrics as any gasoline car on the road while getting better than 100MPG.
In fact, rumor control has it that GM already did so. Remember the EV1? One of the engineers working on that project had to build a tow-behind generator for the EV1 because the battery range on the car would not allow the car to pass GM's required 24 hour endurance track test. Supposedly the EV1 pulled down better than 100MPG with that generator providing power to the car. I say rumor control in that I read about it online, but can no longer find the story I was reading about it. Either it's no longer online, or I'm not searching correctly as I read the article several years ago. Since I can't prove it with a link, I expect everyone else to take it as hearsay because I'm here saying it
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I went to find the article on the car from 1978, found this
70MPH, 100MPG car instead.