You probably need an alignment.
Do you mean that the tire is cuped or worn on the outer edge more than the rest of the tire is.
If that is the case then you need an alignment.
I think cupping is caused by the caster being out of adjustment.
You can probably find some place to check your alignment for free but they will charge you to correct the problem. Between $60 and $80 at most places.
Thats all the info that I can give you.
Hope it helps