Technology

Japanese cars, hybrids aside, aren't efficient. They're more efficient than American trucks of course, but they achieve low fuel consumption only through their small size.

I'm not kidding when I tell you that I achieved the same highway fuel consumption on my mate's 2005 Mazda 3 1.6, 5 speed manual, as I did on my 1999 Chrysler 300M 3.5, 4 speed auto. The Chrysler had just over twice the power and it was a bigger car. And older. AND it was an old slushbox auto. The important considerations: Despite all the extra length, width and engine size, the weight difference somehow was only about 300 KG. And the big engine had a wide enough torque curve that they could allow for really tall gear ratios, it sat at about 1500-1600 rpm cruising at 90 km/h in 4th gear. The Mazda I believe was nearly 2500 rpm at 90 km/h in 5th, but I could be remembering that wrong.

Around town the Chrysler lost more of its fuel economy than the Mazda of course, but then it was a huge ass car with a huge engine - the difference was still nowhere near what one would expect it to be. And of course with a 3.5 exactly when the gas prices were at their lowest in ~2015-2016, well, I accelerated away from every light with a grin on my face so that probably makes up for much of the city fuel economy difference (which I honestly don't remember the numbers for - it varies by traffic density anyway and I didn't drive the Mazda in cities much, it wasn't mine).

I then drove a ~2005 BMW 5 series diesel (E60 530d). Completely blew both of those out of the water with fuel economy. Think I got 4l/100km on the highway when the other two were 6-6.5.

You are right, though. Efficient cars are buildable today, there's just close to no reason to do it in the US where gas is essentially free compared to, say, the EU. It's the same reason people in Gulf states love driving their Nissan Patrols (Nissan Armada in Yank terms). The fuel's so cheap, fuel economy doesn't matter.

The brutally honest truth is that people are just willing to pay more for bigger cars and not just a little bit more, but more than the manufacturing cost increases with size. Make a car bigger, it gets 20% more expensive to produce but people will pay 50% more for it. It's a no-brainer.

You can still buy cars that get <=4 l/100km on the highway, even with modern emissions requirements messing shit up for you (DPF regen cycles cause diesels to use more fuel for example).