its not just the rim size you need to consider,its the profile,ie,the hight of tyre from rim to top of tyre,depends on manufacturer of wheel too.just get low profiles n test a few sets.rear will be fine,just make sure front do not scrub springs etc on full steering lock.as long as the turn your fine,please note that big wheels put your speedo out so you going faster than your speedo tells you
as long as you have the correct profile on the 18" so the rolling radius remains the same(Depending on what was on there to start with )..any tyre company can advise on this.