> the absolute best theoretical efficiency of this might be 70%

While ICE are heat engines with a theoretical limit of 70%, they’re more specialised subsets described by the Otto (gas) and Diesel (… diesel) cycles, which have a much lower theoretical maximum.

Just plugging the temperature ranges into Carnot will give you a Carnot limit of 50%, and using Otto will yield 46% (https://physics.stackexchange.com/a/98992).

Add in that gas engines are not spherical and into a vacuum (losses and delays) and you’re in the 30s.