just bought some 18×7 wheels and was thinking of running a 215/35r18 tire, would I have any problems with that? Car has stock suspension and stock fenders.

 

I'd say the diameter is too small.
OEM 205/55-16" (24.9")
215/35-18 (23.9")
In the 18" range I'd say 225/40-18" (25.1") all around or some add 255/35-18 for the rear tires(25.0"), but wheel width is too narrow as the 225 requires a 7.5-9" wheel and 255 8.5-10".

 

In addition, your spine will not like the 18's as much.

 

do you mean the aspect ratio? cause 18 is the diameter I need for a 18" tire, I want something that will not be too huge but also not look goofy cause it doesnt fill in the gap at all.

 

No, 18" is the inside diameter, but the outside diameter (23.9") is 1" smaller than the OEM 205/55-16" (24.9") tires. In fact, Mazda was going to use 215/55-16" (25.31") tires, but at that time the 215/55-16" tire was just about non-existent, therefore the 205 was used.
I have 215/55-16" in front and 235/50-16" in rear and they certainly don't fill out the wheel well.

 

o ok makes sense so with a 18" what size tire would help fill out the wheel well but not be too big?

 

205/45/18 would be only slightly larger in diameter than stock and be a good for for those wheels, 225/40/18s would also do the trick but would bulge over the wheels a bit.

I wouldn't anticipate any fitment problems with those wheels, they should fit in the fenders like stock.