My sister is getting married this weekend she really wants the grooms cake to have a 3D motorcycle tire slinging "mud" on the Wedding cake. Here is an image of what she wants....  

I have convinced her that I do not want to carve it so she has suggested rice crispy treats, but I have never worked with them before. Can anyone provide me with a tip or two!? Please?  :)

I know I will cover in fondant but how to get it to wrap nicely, without tearing and then to make the indentions and where will the seam be? 

I've only used rice krispy treats on a small graduation cap, but I can say you'll have to smooth it out or crumb coat it or something before covering with fondant. If not, you'll be able to see all the little bumps from the rice krispy treats.

Here are a couple of threads about rice krispy treat sculpting.

http://cakecentral.com/t/231571/can-someone-please-shed-some-light#post_2376311

http://cakecentral.com/t/623004/ill-never-use-plain-old-rkts-again

I would imagine you'd roll out the fondant a little wider than the tire - do the tread and hub cap part in two different sections, the gray 'rim' would cover your seam- and maybe only have to cover as much of the tire that won't be hidden with the mud?

For the indentions, maybe use something like the veining tool?

Just my thoughts :)