I actually looked it up.
A late 19th century engine had a 26% efficiency.
1960's engines had ~34% efficiency and in the 2010's it was ~43%.
So the same fuel consumption would give you 26HP in 1900, 34HP in 1960, and 43HP today.
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.
Diesel's engine used 317g/kWh.
So the amount of fuel used has not even halved in more than a century.