The question was not really well defined. But just looking at drag force, there are a lot of things that can be done to reduce it by more than 10%, they're jus

The question was not really well defined. But just looking at drag force, there are a lot of things that can be done to reduce it by more than 10%, they're just unpalatable, impractical, or simply haven't been implemented. Also a lot of energy is lost to heat, even in electric cars if you're looking at the whole car and not just the aerodynamics.