Tires on my 2016 Mustang GT
 

I have 20" wheels and would like to know what is the maximum size tires that I can install on my GT?

Go to the Tire Rack site and play around with different size tires.

I believe your stock tires are 265/35 and the rims are 9" wide. You will likely find that the tire manufacturers won't recommend anything wider than a 275 on a 9" rim. A 285 will likely fit but won't be optimal for handling and you may find that tire stores won't mount them from a liability standpoint. Also need to consider how much error in your speedometer you are willing to live with.

If you want to go bigger you will need new rims.