I have been asked to make a stack of truck tires with three (3) on top of each other and one leaning against the stack...What would be the best way to lean this tire without it falling or slumping down????

I think I would make the leaning tire out of RKT. Not sure if this helps, but I can't see how a round cake could be on its side without making some sort of holder for it.

yep. fake.

Or you could use a big sheet cake and cut it in strips, layer them, then carve the tire out of that. You would have to use a board in the middle of the stack and do some doweling. The rkt would be easier, but just in case you didn't want to do that, this is an option.

You can lean the third tire, made from cake onto the two other ones with a piece of styrofoam behind it very discrete also covered in fondant and well secured with dwels and skewers.