I just bought a 2008 4.8 X5, but need advice on a winter set up. I am currently running on the 20 inch sport rims with the SP Sport Maxx tires. Is it possible to just switch the tires to a winter season tire, or do i need to purchase a 18 inch wheel and winter tires. Also If i switch from 20s to 18s will this cause any issues with the speedometer, or traction control, etc?

Thanks