I am purchasing an X5 that has 18' tires. I have snow tires and wheels that I used on an X3 which are 17". Can I use the smaller winter tires on the X5?
Snow tires are traditionally narrower than summer tires, to get through the snow better. You can use 17" wheels, but you should attempt to keep the overall tire circumference the same as on the 18" tires. Use this tool: http://www.1010tires.com/tiresizecalculator.asp
