Hi,
We took at trip from Phx Az to Las Vegas (about 6 hrs) when our Poppa Mack was only 8 months old. We have an SUV, that we bedded in the back and he rode back there. We had been taking him for short rides to the petstore, vet, etc so he was somewhat ready to roll. We had people in the back seat that could keep an eye on him to watch for any symptoms. We made sure we stopped early for potty breaks and to give him a chance to get his "ground legs" back after the motion of the ride. He stood for the first 2 hours and would not lay down, but gravity finally took over and then he laid down. The trip was smooth, not problems, not accidents and enjoyed taking him to the LV~EBD Show.
Start off on short trips, and then work up.
BTW, we once shipped a puppy from Az to New York state and it was a 9 day van ride. Of couse the puppy was SOOOOO cute, the transporters held him the entire trip so I'm not sure if that counts or not?