[BobbyJo]

Source?

Edit: Also, you're confusing theoretical and practical efficiency. For instance, a V8 engine can be 30% efficient in absolute terms, but if you can use an I4 for the same application because you now have a Turbo, your real world efficiency is way higher. This is exactly what we see happening in performance cars an large trucks: big displacement being replaced with turbos and hybrid systems.

[Gauge_Irrahphe]

Wärtsilä 31, which claims to be the most efficient engine ever built, claims 167.7g/kWh.

Diesel's engine used 317g/kWh.

So the amount of fuel used has not even halved in more than a century.