[Superfly_M3]

Hello,

Today I thought that I was going a little slower than was indicated and decided to check with GPS. Mind you its only an iPhone app, but it can't be off that much. I checked and my jaw dropped. Going indicated 80km/h GPS showed 73km/h. Going 140km/h GPS showed 129km/h. Thats 9%!!!

I just switched to 235/40/18 winter tires on OEM winter package rims - they have at least 60% thread left. My summers are 245/35/19F and 264/30/19R.

The diameter for the tires are:
235/40/18 - 25.40"
245/35/19 - 25.75"
265/35/19 - 26.30"

What could be wrong? Can it be fixed at the dealership through programing?
Where do our cars pick up the speedometer reading front or rear tires?

Thanks in advance.