Above 40mpg you need hybridization. That said modern sedan hybrids get considerably more mpg these days than 45...Like the Ioniq hybrid that gets 58mpg combined at the rather reasonable price of 22K base.
The other thing that gets overlooked is there's a lot of vehicles that get a LOT better highway gas mileage (compared to how people normally drive them in non-highway use). Hybrids get almost the same in both situations (heck the Prius gets better mileage city driving).
Based on my limited understanding of the auto industry, it seem like as far as using fuel more efficiently in the engine itself... There really isn't much we can do there at this point. There has been practically nothing done in the last 10 years to make engines more fuel efficient, and I have already experienced that myself. My 2006 Acura still gets the same gas mileage as a ton of NEW vehicles today.
So what does that leave? Cheaping out and going for lighter-weight so that the engine doesn't have to work as hard to bring a 2 ton vehicle up to speed, but rather only a 1.5 ton vehicle. You get the picture. What does that mean? Definitely will raise an eyebrow as to how durable the product is, and also how good it is with crashes. You're basically just boiling it down to replacing what was previously steel/metals with plastics.
Good for the automakers and CA here I guess though. Personally I think it's a waste of time. If it isn't hybrid or all-electric than it's a stupid discussion in the first place.
If you posted more like this I wouldn't have nearly as much of an issue.
It is limited, but you admit so and that's a nice thing to do.
There are things we can do, and there are things we have done in just the past 10 years to do so. Direct injection for instance was fitted to gasoline (it has been used in diesel engines for some time, and is part of the reason they got so much better gas mileage) which boosted fuel economy. Mazda has some new tech (I can't recall exactly, I want to say it is some special way of sparking the ignition which lets them use less fuel; look it up they brand it Skyactiv-X), Nissan has some new combustion chamber design (it allows for variable combustion chamber). Greater valve timing control (Koenigsegg for instance uses solenoids to replace camshafts which offers greater control over the valves, but they're expensive so most car companies aren't going to use them). Better use of turbochargers (enabling them to get higher outputs from smaller displacement/engines). They're researching laser ignition to replace spark plugs. Formula 1 has actually gotten engines to be about 50% thermally efficient (most cars are like high 30s these days I believe, used to be like 25%, I think some certain ones like maybe the Prius' gas engine and other Atkinson cycle engines and the Mazda one I mentioned are like low 40s). There's been improvements. They're not always easy and they're not always cheap (if everyone could afford $100k vehicles, we could probably have 100mpg as the norm, of course that wouldn't matter if people instead chose to go for less efficient more fun vehicles - and that's an issue at all prices is consumers choose fun or fast over practical or economical).
That's not true. Basically none of it is true. Its actually not cheap to get lighter (aluminum costs more than steel, carbon fiber and other materials cost even more; but they're using a mix of materials on cars to try and maximize the right material for different areas - maybe steal monocoque with aluminum body panels, plastic bumpers, etc). Reliability (of the mechanical powertrain) on cars has actually improved (its generally all the electronics for comfort stuff that makes cars seem less reliable today - but even that is changing). Certainly if we all drove tube framed fiberglass bodied cars with like 150hp engine, we'd get good gas mileage, but we'd be miserable most of the time while driving.
Safety has improved (its plateaued though because the weak link in the safety chain is the drivers, where they're driving faster and with more distractions than probably ever; point being you can make the car safe, but if idiots are gonna tailgate going 85mph well accidents are gonna happen, or if they're distracted texting or talking on cell phones; but that's why they're introducing autonomous technology which in the interim is bringing automatic braking systems that can brake before a human could even react to start braking). Safety improved because it was mandated. And yet even though cars weigh more now, their efficiency has also gone up. And not only did both of those improve, but new cars are loaded with features and creature comforts (seriously, even "economy cars" now have like heated leather seats and stuff). But that does make it tough to make cars lighter because people expect the modern comfort and tech, and they expect safety (although they'll look the other way often if they're allowed to, much like helmet laws, where mandating it protects people from their own stupidity because otherwise they'll think it won't happen to them), and they expect power as well. There's been some argument that if we stripped away the safety and comfort that drivers would naturally drive slower and more cautiously, but I'm not sure how true that would be.
The biggest issue is that consumers keep actively choosing less efficient vehicles. They want horsepower and SUVs. Electrification helps with that but right now its expensive and if we banned ICE and mandated electric cars, we wouldn't be able to satisfy demand. Heck, I don't know that we could produce enough to just replace say the sales of just the Ford F-150. There's limits because of the materials needed for batteries (there's a race to develop new batteries that can run on less rare/costly material, like replacing cobalt - er I think its used more in the electric motor but its still something that needs addressing with electric vehicles).
That's exactly what this pushes though. By mandating higher fuel efficiency and emissions standards, they have to go to hybrid and electrification to achieve them, without explicitly banning ICE.