hmm what kind of car? some things you're going to have to consider is that every tire will look slightly different on the same wheel. you have some tires with stiffer sidewalls, others with rim guard that make it appear wider, the weight of the car will also affect how it looks. personally I'd fit the largest tire you could on the wheel, I had a 275-35-18 on my 18x9's but once again it all depends on the type of car