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DCL
Most of us
But why does it matter?
Because as speed increases, air resistance increases too, but not in a linear fashion (more details about Aerodynamic drag this can be found on Wikipedia) and quickly adds up to a substantial force that the car must push against. This means the engine must work much harder than if the car was moving in a vacuum, burning some fuel just to move the air out of the way.
If you want a good illustration of this, try putting your hand out the window of a vehicle moving at 30 MPH and 60 MPH. It'll be quite obvious that the force is more than twice greater at 60 than at 30 MPH.
Easy Tips
That's why it is not recommended to roll down your windows at highway speeds; the extra air resistance makes your engine work harder, burning more fuel. It is also why you shouldn't keep unused roof racks and bike racks on your vehicle (if you use them, that's fine, but I know people who use them maybe once every few years).
But most importantly: If you need to buy a vehicle, get one that looks like it is slippery through the air and not a boxy one (well, just get the car with the highest MPG that fits your needs, and that will probably also be a very aerodynamic model, since there's a huge relation between MPG and how slippery a car is through the air).
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