My dog does that too! I'm not really sure if he still does that though because it was a long time ago that I took him in the car. He'd do that on the normal speed roads but then for some reason when we went on the highway, he didn't do it which is ironic because the cars are going faster and they are louder as well. Maybe you could get a muzzle type thing or a shock collar. I'm not really sure what you could do. If you don't know what else to do...you could always ware him out so he sleeps for the ride. It's funny though I thought my dog was like the only dog that did that...but apparently not.
Well Good Luck!