I think IntialD has the right idea. It seems that running the 22 tooth side pulley will help the tires wear evenly, that is if you have the correct split. I guess the split needs to be 3 or 4 mm; which I don't know. I wished the manual would have explained the tuning options the R40 has.
Anyhow, people seem to run the front overdrive set-up mostly to get even tire wear. I can see where running the 22 tooth pulley would make the front tires pull harder than the rears. Also I can see where you need a split so that both front and rear tires have the same roll-out. I guess the even wear comes from the fact that now the front tires are working harder and so they are wearing just as fast as the rears.
I wonder,,, does running the 22 tooth side pulley make the front tires wear faster or does the wear on the rear tires slow down? One or the other has to happen in order for the tires to wear out more evenly.