well you need to look up more detail of the law, its not saying every vehicle has to get 35 mpg, it says that the avg fleet economy has to get 35 mpg, meaning you can have some that gets less as long as you get some that are more, meaning lets say they can make the V8 cars and trucks only get 30 mpg then they only need the same amount of cars get 40 mpg or better, so just the avg mpg of all the vehicles in an automakers fleet has to be 35 mpg, doesn't mean every vehicle has to be 35 mpg or better
also the law wouldn't take effect until 2020
but to answer your question, nobody will buy a 4 cylinder sports car that should have a V8, they shouldn't even have 6s in them, and just so you know they make V8s now that can get mid 20s mpg, even the full size trucks and SUVs, even the corvette can get 25 mpg, also the new camaro SS with over 400 HP can get 25 mpg on the highway