55 was a response to the oil crisis but it also reflected automobile engineering of the time.
Most of today's cars are significantly less massive even if their size is larger.
Similar-sized cars are probably lighter nowadays, but the overwhelming shift towards SUVs holds back the average mass.
Now I'm misty-eyed, thinking of my beautiful LTD... a completely inappropriate car in 2019, but so much fun to drive.
Isn’t this a function of wind resistance physics that can’t be solved, just mitigated?
As driving automation increases and the threat of climate change and CO2 emissions looms larger, automated cars could drive at current speed limits without getting impatient like a human driver, improving both efficiency and safety.
But it only matters on highway where the bottleneck is due to aerodynamism, at lower speed you might be less efficient at 20mph than 30mph for example.
Isn’t this a function of wind resistance physics that can’t be solved, just mitigated?
Eventually air friction is the completely dominant force. So at highway speeds, absent some rare gearing issues causing inefficient fueling, faster means less efficient.
This is also why the fastest "hypercars" are pushing on order of 1000 hp.
At lower speeds, all sorts of other issues kick in (and idling is its own issue)
https://en.wikipedia.org/wiki/Production_car_speed_record
The RUF (Porsche) needs 460hp to break 213mph. The next record breaker needs 618hp to break 221mph. The 241mph record is made with 806hp. This continues incrementally until the current record holder, which needs 1350hp to hit 278mph.
It took three times as much power and three decades of computational advancement to build a car that could go a mere 65mph faster than the 1983 record breaker.
Side note: the listed horsepower numbers before 1980s are basically made up. You can safely assume SAE J2723 horsepower (what cars use today) is ~60-75% of published value for cars older than 1980 (and about ~80-95% for cars between 1980 and 2005).
If we move to a more infrastructure style car transport where you just subscribe or have access to (depending on where we end up on the public-private spectrum) there's probably a lot of gains we can still make that will make cars much uglier (to our current eyes) but more aerodynamic.
If I hit a hard barrier, I'd prefer that the car break and disintegrate and dissipate all that energy, as opposed to transferring all that energy into me as the occupant.