It isn't the same amount of fuel. If the car's engine is going to power something else besides the car, it is going to have to work harder.
Imagine the car is running in a vacuum and gets 30MPG. Now, add normal air resistance. It will not get 30MPG anymore. Now, add something else that further increases wind resistance. The car will get even fewer miles per gallon.
Imagine the car is running in a vacuum and gets 30MPG. Now, add normal air resistance. It will not get 30MPG anymore. Now, add something else that further increases wind resistance. The car will get even fewer miles per gallon.