I am making a monster truck cake for my nephew next weekend. I was wanting to make a 2 tier cake with a monster truck on top and one going up the side. I was wanting to make tire marks around the cake. How would i do that? And how do i make sure the tiers dont fall over? Should I put it together when we get to the place of the party?

I have made a couple caks similar to what you are describing. To mae the tire tracks, you can make the tracks with a toy car/truck that has treaded tires. Dip the back wheels into black food coloring and roll them where you want the tire tracks. Here is someone else with the same idea for wrapping paper, but it will give you a visual of what I'm trying to explain. http://www.tonyastaab.com/2010/03/tire-tracks.html
Hope this helps.
Happy Baking