19's on the front & 20's on the back??
Again, it is strickly looks. Going 20s in the rear just leaves so little rubber sidewall to take the undulations. You end up getting wheel hop or just smoking the tires, PLUS you will likely go taller overall which will also make your car slower.
To put a similar setup on a car that is not expecting different diameters will probably throw off the electronics. I am not sure what systems like ABS would think as it sees the wheels effectively moving at different rates. The other way to do it is to reduce the sidewall in the rear so that the total diameters are the same, or close to it. Personally, I would not think this would look very good.


