What happened to gasoline in the mid 1970s?

Right! WHERE is the measurement for unburned fuel? There is none. Know why? Because the gubmint isn't interested in making vehicles more efficient. They got there once. Remember the little cars from 89 to about 2003? They kept getting better and better mileage until.....wait for it.......the GUBMINT stepped in and made automakers TUNE for ethanol. We had an 02 Corolla I bought for Kitty brand new. Little 4cyl, variable valve timing....all that "new" stuff. That car knocked off consistent 46MPG on the highway LOADED with Kitty, me, our son and stepdaughter packed SLAM FULL of 2 womens' worth of crap. Still.....46MPG. Then long about 2003, all that went to hell. You don't see those mileage ratings very much now. Before that, they were getting up there. Some of those Geo Metro cars got 60 plus.

If some of yall think this is about pollution and clean cars, you're a FOOL. It's always about the money. It always has been. My Uncle is probably one of if not THE MOST respected economist in the world. He's written many times about the uselessness of gubmint regulation and in particular how useless the EPA is. He's right, too. They don't do anything they were created to to. But they do make the gubmint a lot of money.

What money is in ethanol that is motivating the government?

It goes back way farther than 1989

the EPA rated the 1.3-liter four-cylinder 1984 Honda CRX at an astonishing 68 MPG in highway driving

In an effort to standardize production, Honda used the 1.5-liter four in HF (High Fuel economy) versions beginning in 1985. Thanks in part to a change in the EPA rating system, the car's fuel economy rating fell from the original 68 MPG highway to a still-impressive (and somewhat conservative) 42 MPG highway. Owners frequently reported highway fuel economy in excess of 50 MPG, and CRX fans are quick to point out that modern hybrid automobiles, for all their expense and complexity, struggle to top this number.

https://www.hemmings.com/stories/2013/06/24/lost-cars-of-the-1980s-honda-civic-crx

We are 37 years later and cars are still not doing way better. They should be doing way better.