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

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.