As others have said that's not surprising, thermodynamics puts a hard limit on efficiency at around 30-40 % in practice (in theory you can get higher [1]), the rest will be turned into heat. As a half serious remark a car running e.g. with mechanical energy stored in a flywheel could be similarly efficient as a battery-run car. In addition it could almost never flip due to conversation of angular momentum in the flywheel :D Energy density and other practical aspects make that form of energy storage not very suitable for cars though (there actually were some uses in streetcars, buses and other vehicles).

1: https://en.wikipedia.org/wiki/Thermal_efficiency