I started out with yellow and chocolate cake mix. Thank you, Betty Crocker!
I baked one 9x13 pyrex dish chocolate cake for the base, a small loaf yellow cake for the main truck part, and a miscellaneous IKEA pan yellow cake for the truck cockpit. Using the main truck part as a giude for size, I cut the rest of everything to scale (somewhere My Husband is still saying thank you for cake scraps).
I used a piece of foam poster board, wrapped in aluminum foil, for the cake to stand on, and a piece of floral foam, also wrapped in aluminum foil, to hold the base of the cake up. I covered the base with chocolate frosting, then attached the wheels (two oreos stuck together with frosting).
The rest of the truck was vanilla-ed. The decorations were a bunch of candies from the cake decorating aisle in my grocery store.
As usual, it was sort of touch and go in different places (I couldn't figure out what to use for wheels for a long time), and transporting it to the park where our friends were having their farewell party was really emotional for me (no kidding) because I was incredibly worried the whole thing would slip off the seat (though it was buckled in) or fall apart or melt or...
But we made it, and people were totally impressed with me. Just the way I like it.