I've found the 18's worked perfect for me, 1 MPH difference in the speed read out as compared to my Android nav. I have Pirelli ice and snow tires and in WI make all the difference in the world.

As for the size, I don't know as JCL says, 17 or 18 would make any difference. I would think the 17 would be off a bit more on the speedo. JMO

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2009 E70 White w/black int
2006 E53 X5 4.8is Sold
2000 SVT Cobra R #193
2003 SVT Mustang convertible