Where is 3% tire size difference measured?
 

I think I am on the right track with this, but when the tire size calculator says my new tire size is 2% slower then stock for the rears and fronts are 2.3% slower, does that mean that I am on the safe side for not having VDC go nuts? I searched but couldn't find a lot of info on what the 3% difference meant or where it came from.

bump for an answer. I'm thinking about getting the m310s with 305/30/19 tires and it says that I would be -3% difference. So the speedo would read faster than i would actually be going.

Don't know how this affects VDC but in the M310 thread, no issues were reported with the tire size i'm mentioned above.