Something to add to what Maxx has stated....... I would also like to state the my 17's come pretty close to the top of the wheel well,,,as you can see from the picture below....and that's with no one sitting in the car,,,now if you start adding people of course the car is gonna be even lowerer...I've had four people in the car and I haven't had any rub but I don't know if with the 18's you will but it's just a thought to ponder over....maxximtl said:It should work, as long as you can stay with a 7 inch width. I believe a 7.5" width would rub. You'll have to pay extra close attention to the offset too.
You don't want to go any wider than 215 either. You'll have very limited tire choices at these sizes.
Personally, I think 18's are a problem waiting to happen. With such little sidewall, the rim becomes very susceptible to damage from potholes and curbs. Also, 35 series tires are not known for their ride quality. I guess it would look cool though....if your into that sort of thing.
The Sentra doesn't have very big wheel wells. 16's or 17's fill them out nicely. An 18 might be too big, and look out of proportion with the rest of the car.