Biggest advice I can give you is when you hit the desert stay hydrated so pack extra water. There are thousands of unforgettable roads to see between Tennessee and California on your route. I know RT66 kind of follows I40 across the top of Arizona where you will be in higher altitudes along the way. That will get you close to the grand canyon make sure you drop down Flagstaff into Sedona it's worth the trip just to see the red rock formations. When you get to Kingman you can turn north on 95 and you can go across Hoover dam or bypass it then right into Las Vegas. When you hit California watch out for the idiot drivers.