Can you source that? Diesel is only 13% more energy dense than gasoline [1] so the difference between the two fuels isn't huge.

I suspect that modern (last five years) turbocharged gasoline engines are probably approaching diesel thermal efficiency, but I don't think that it's correct to say that they generally surpass it. The gasoline Ford EcoBoost is 33% thermally efficient while a BMW N47 turbo-diesel is 42% thermally efficient, as an example [2].

[1] https://afdc.energy.gov/fuels/properties [2] https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio...

The fundamental difference in how the engine operates by throttling fuel only instead of air and fuel accounts for a large fuel economy savings

Fuel is sold by volume, which is why volumetric fuel efficiency is desirable to the consumer

Fuel is sold by volume and fuel type; diesel is about 25% more expensive per gallon than regular gasoline where I am.

Correct - where I am it is cheaper most of the year, a bit more expensive in the winter.

And it is 10% cheaper than gasoline where I am (South-Africa)

Yes, but measuring miles per volume of fuel and setting increasing targets was a big focus of reducing petroleum dependency since the 70s.

The focus has more recently shifted to reducing overall emissions of CO2 and other harmful gases and particulates, which makes diesel much less appealing.

I don't think any car buyer has ever looked at Calories per litre of fuel as a relevant metric for purchasing.

People that buy cars almost exclusively care about cost of fuel to move between A and B.