not to shoot you down or anything...but if your hoping for power gains with it, there are none to be had.
normal road speeds never even come close to the velocity of the air being pulled into your intake. the calculations were done once, and you'd have to be going somewhere in the area of 180MPH to see any amount of improvement.
Actually, it's not sufficient to match the intake air speed, which isn't constant anyway.
You need to be going fast enough to make dynamic pressure, which is easy enough to calculate.
It's 1/2 * (air density) * (net airspeed)**2
The net airspeed is the difference between how fast you are going in still air and the intake airspeed.
If you want 10 PSI boost with the engine OFF, you need an airspeed of 340 m/s at 20 deg C, which is DANG close to the speed of sound at sea level. With the engine running at peak power, the intake velocity in an ideally designed engine approaches (but doesn't exceed) the speed of sound itself, so you need a vehicle that goes near Mach 2 to get 10 PSI boost.
My two cents here... the only reason one would do the ram air thing is for a cooler air charge to the engine. With that being said, your engine already draws cool air from in front of the grill... unless you have a 4X4 and removed it for deep water crossings.