Are 20mm fenders a big difference from the stock fenders? Will it require me to buy new tires/rims to make it look right?

I love the stock, but i also like the vents that some fenders come with. but it seem that it is atleast 20mm, so im just wondering if it makes a big difference. In no way i want to make the car any wider.

 

There is a noticeable difference (just over 3/4") but not that much. If you looked at the car from the front, you could really tell (if your still using the stock wheel width) but not that much from the side.