Hi

I have got a good price on new winter tires to my RS. Michelin x-ice3 225/45-18

I know that it should be 225/40-18 but will it be any problem with 45 profile?

I haven't lowered the car for now, but until next summer I will fit Eibach springs.

Do you think I should pass the deal and buy some other in original size?

A 45-section will have a ~4% larger rolling diameter. This means that your speedo will under-read, potentially rendering the wheels illegal. 45's will fill out the arches more, and will have a small affect on geometry/alignment.

215 45 would be better and possibly cheaper. Only a 2% difference

In Norway +/-5% variance in rolling circumference is allowed with some additional requirements, like no conflicts at max. turn and full suspension. I assume it's the same in Sweden. The problem you can encounter is when you lower the car, the tyres will be 11 mm closer to the wheel arches. So you must be careful with the tyre/rim match related to ride height.