Have we? Modern diesels easily get 60-70mpg(49-58 US mpg) without even trying, and modern petrols are not much worse - I recently drove a small VW Polo with a 1.2L I4 90bhp petrol engine, fully loaded with luggage, and averaged 54mpg(45 US mpg) over 600 miles. Even my own Mercedes-AMG with 381bhp can easily average 32-35mpg(26-29 US mpg) when driven on longer trips.
The diesel comparison is perhaps not entirely fair. The density of diesel is about 833 kg/m3 compared to 740 kg/m3 for gasoline. This gives diesel about 13% higher energy density per volume.
Sure, that's why I gave two petrol examples as well.
It is incredibly slow and small even by today's compact car standards. We have made a huge amount of progress.
Sure, but that car took 20 seconds to get to 60 and only weighs 1300lbs. It probably offers little more safety than a motorcycle. Neat piece of history though
It weighs about 600kg and has a 1 liter engine that makes about 50bhp. You can run a car of that size and weight on solar power in 2019.