[Otto]

When designing front wheel deflectors, you might want to consult the Hucho book: In that there is a pic of a car (Calibra, perhaps) in a wind tunnel, and it shows that centerline airflow deflects at about a 60 degree angle outboard toward the front tires. So, the wheel deflectors should be canted inboard correspondingly, i.e., pigeon-toed so as to meet the incidence of the air as it actually impacts the front wheels. In other words, an air molecule in the free stream may splatter against the inner sidewall of the front tire at about a 60 degree angle, meaning that the effective frontal area of the tire is much greater than if the air were hitting it squarely on the front tread. So, the deflector should be installed so as to fair the flow as it actually travels to and past the tire. Otherwise, the tire acts as a wall to partly block the airflow from escaping from under the car.

Easy way to check on your car: Tuft testing, with the tufts mounted on inverted Ts, showing flow 1" to 6" below the level of the car nose. Then, have a friend with a camera shoot pics from an oblique position at highway speed.