Can I go 3 cm thinner on my tire width and it mount ok? I have 255 40 17s and I want to put on 225 55 17s

Its for an Audi A6. Winter Tires

Sure can without any problems!

This tire is over an inch narrower. This may cause uneven tread-wear. The best advice is to get a rim an inch narrower. The tire shop will have references as to what size rim the tire manufacturer recommends for that tire.