I picked up an 2001 convertible c5 base.
18 x 8.5 +56 offset fronts18 x 9.5 +50 offset rears
Both have 275/40r18.
What size Rear tire would make the proposition even. I think since I have 275/40r18 up front should I run a bigger tire in back and what size or should I leave the 275/40s in back and get a smaller tire in front?

 

Welcome to the forum. Do you know what the OEM tire sizes were? A bias towards wider tires in the rear will tend to give you more neutral steering if you're starting with understeer which most cars have from the factory. You might check with Tire Rack because I believe they can tell you maximum tire sizes or different cars.

 

Factory size is
17x8.5 +58. Backspacing 6.5 tire size 245/45r17
​18x9.5 +65 Backspacing 7.35 tire sise 275/40r18

 

Personally I'd be inclined to put a narrower tire on front because you're probably going to have an oversteer condition as it stands now. Of course if you're into drifting then that might be okay. 😉

 

What size tire for the 18x8.5 front should I get to go with the 275/40r18 in the back? 265/35r18?
front will be
18x8.5 265/35r18
rear will be
18x9.5 275/40r18

 

I guess it depends on if you're going for looks or if you're going for specific handling characteristics.

 

Just looks really. It's just a cruiser. I am not auto crossing or anything like that. I want it to look the best lowered

 

I would recommend sticking with the stock wheels and tire sizes if you're planning on lowering it because that gives you the most clearance.

 

Stock front is 17x8.5= 25.7x9.5
18x8.5 equivalent is
245/40R18 = 25.7x9.6

on the front now.
275/40R17 = 25.7x10.8
18x8.5 equivalent is
265/35R18 = 25.3x10.4
275/35R18 = 25.6x10.8
285/35R18 = 25.9x11.2

rears
275/40R18 = 26.7x10.8R18

 

Another thing to consider is that the shorter profile tire will give you quicker handling but also a harsher ride.