Good thinking- Just take it one step further to understand what's happening.
For a given speed, you are displacing x cubic feet of air per second. The air striking your vehicle provides all sorts of unwanted forces- Lift, drag, and with some cars, downforce. Increase your speed, and you increase the amount of air you're displacing per second, thus increasing all of the above forces. Some are desireable, some are not.
For cars, you'd like to minimize drag, just so more of the power your engine puts out can go towards moving the car rather than fighting the wind. Now, cars will always have some coefficient of drag, so you just have to deal with it. Minimize it by all means, but you'll still have some.
Lift and downforce- These are touchy forces to contend with, at speed even more so. Sometimes you can have situations where your lift actually becomes a significant fraction of the weight of the car, taking your power source (the wheels) away from the ground. This is obviously not cool, since you end up decreasing the stability of the whole vehicle. You can counteract lift by lowering the car (allowing less air to get under the vehicle) or by providing downforce.
Here's where ricers haven't a clue. Your power comes from your front wheels, so, naturally, you'd like to keep those wheels on the ground. So, provide some method of downforce in the front of your car- An air dam, or valance. Best example- The Visteon front air dam. It cuts the air as the vehicle moves and disburses its force towards the ground as it moves. Your front wheels like to stay on the ground, where they belong. Spoilers in the rear of the car are about the dumbest thing possible for FWD cars (hence the clueless ricers who pin 8-point bucks to the back of their Honduhs...). You provide rear downforce, right where the wheels that aren't doing a damn thing are sitting, shifting the weight away from the drive wheels.
Of course, you don't really have to contend with these forces on a large scale until close to about 100MPH or so- It's crucial for NASCAR, BTCC, and F1 where speed is paramount.
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