[Big Dave]

Aerohead is right about the SAE recommendations, but there are always exceptions.

My experience:
I made an air dam out of nearly indestructible (and hard to fabricate) 3/8" ripstop conveyor belting.

I got greedy and (first iteration) left it 3/4" above grade. From a fuel economy standpoint, it worked like a champ. But it dragged a lot. No damage, just that annoying sound.

Next try, trimmed it back to 1.5" above grade. Again, excellent MPG performance but it still dragged.

Next try 3" above grade. More or less same result: Good MPG, dragged a lot. No damage, just annoyance.

This year I trimmed it back to 8" above grade in front of the wheels and 12" above grade inside the hanging-down suspension parts. Big mistake. MPG dropped off but at least it doesn't drag or look so dorky.

If it doesn't improve MPG, what good is it?