This topic has been kicked around a lot on the forum. There are physical restrictions on minimum and maximum wheel size and width, which determine the possible sidewall aspect ratio of your tires. You must buy wheels that fit over the brakes, and they must also fit within the wheel wells -- those are your limiting factors. Once you have figured out what those wheel sizes are you can identify the tire combination that gives you the tallest sidewall.

Note that sidewall height is not the same for every 30, 35 or 40 aspect ratio -- you must also consider the tire's width in the formula. In general, to maximize sidewall height you need the smallest wheel that fits over the brakes. If you go looking for "Crossfire" wheels you won't find one under 18", but you may find a smaller size looking for wheels intended to fit the comparable Mercedes model.

There is not a lot of documentation regarding odd-size wheel fitment for our cars. Try before you buy or you may be sad.

Happy motoring.