I am doing an 18 wheeler and I want it elevated so it's not level with the tires. I was thinking of building a piece of plywood with 2 separate platforms. One for the cab and one for the trailer. If anyone knows an easier method I am open to suggestions.

Let me find my instructions for my fire truck...this will work for your 18wheeler.  Or at least give you something to read and compare to other input.

Ok...here you go:

http://cakecentral.com/t/762618/tutorial-or-video-for-building-a-fire-truck-cake-support-and-structure

An 18 wheeler is in 2 parts..like a fire truck..the back is just longer.    Let me know if you have any questions.   I did mine in butter cream...fondant would be much easier. 

thanks..definitely doing fondant

HI, will this help you.

http://culinarychat.culinary.net/keep-truckin-how-to-assemble-an-18-wheeler-cake/

On first picture of truck and

horse trailer I cut  styrofoam then covered it with dark grey fondant. Then cut foam core same size as cake so it appears to have an overhang beyond the base. If you don't cut a circle to inset the tires you can put a fender over the tire to give it that same illusion. In second picture I just frosted base in grey color  AKA truck on a much  lower budget and price point.

I LOVE the first one. The second one is good but not the amount of detail I would like. Thanks for the picture!

I want to start this but am having trouble figuring out scale. I have a 2 1/4 circle cutter I want to use for the tires. I can't find a tutorial for that