the last few weeks I've noticed that my front tires where getting worn faster than my rear. I see that on the outer edges of the tires is gripping pretty hard. but I do not feel anything unusual while driving....
the car has only 15K miles no mods. is this normal for a rear wheel???

A lot of the newer nissan models ride with positive camber which wears the outside of the front tires. The recommended rotation is every 8000 miles or so. This seems to make a set of four tires all wear out at about the same time..or so says my eyeball estimation of the treadwear on mine with 12k miles or so.