What's the best ratio and/or recipe to make tires to stand up on a 3-D John Deere tractor cake? Do I need to make them early and let them dry out some before covering in fondant?

I'm doing a 3D JD tractor next week also. I've always used the pre-made RKT that are individually wrapped and find that they work just fine. I personally don't need the added labor of making the RKT! Just make sure to make them very compact. Otherwise they will come apart. Also, you can cover the RKT "tires" with melted white chocolate (candy melts will work) or BC before covering them with fondant. This fills in the nooks and crannies and smooths everything out first.
Happy caking!