I had a rock fly up and over from the rear of the vehicle and strike the inside of the windshield while driving on a gravel road. MT's throw rocks everywhere.

 

I just installed an air deflector on my JK, I will take some pics to show it to you all soon.

 

If the deflector doesn't work on small bugs, it isn't going to work well on rocks. Mine didn't. I got one after one rock cracked the windshield. I ended up taking it off because it didn't do anything except increase drag.

 

I wouldn't imagine those deflectors work too well on rocks. Rocks of decent size are going to be too heavy to have their direction changed all that much by the air current off the deflector.

Windshield chips are just one of those "Jeep things." Best way to minimize them is try to keep a good distance from other vehicles, especially trucks.

A good idea above was full glass coverage through your insurance.