There's not much more you can get out of current fuel blends. Methanol is potent but very expensive to mix, the even more expensive nitromethane is so powerful

There's not much more you can get out of current fuel blends. Methanol is potent but very expensive to mix, the even more expensive nitromethane is so powerful it strains the science of metallurgy and thus is some of the most dangerous stuff to handle, and hydrazine is so energy dense but so unstable it's used as rocket fuel. Porsche's E-Fuel is just barely more efficient than gasoline because of the density at point six megajoules a kilogram, versus point five four for one hundred octane gasoline. The major issue is parasitic drivetrain loss from having to push the cylinder which pushes the cam which turns the flywheel which friction fits against the clutch which friction fits against the transmission flywheel which turns dozens of metal gears which rotates the driveshafts. Direct drive implementations where the engine's output shaft is mated directly to the axle are much more efficient and have been used in massive ships, but are entirely impractical for use in passenger vehicles. There's the Otto, Atkinson, Wankel, Scuderi and diesel cycles for liquid fuel combustion, and all have tradeoffs leaving them competitive with eachother instead of having a clear best solution thanks to everything else required to transfer the power from the initial combustion. In short, we've hit the mechanical engineering dead end for massive efficiency gains with ICEs. It's all chemical from here, and thus gets much more expensive to run.