[oldE]

Pete,
Tire sizes are correct. The sizes for 16 &17" are both on the sticker on the B pillar. I have talked to a number of folks ( other VW and BMW owners as well as service people) about this. One theory goes something like: the speedo reads high so you have a tendency to keep close to the actual speed limit. I find that to be unlikely, as one tends to drive where it seems comfortable. As I think I mentioned in my O/P, the odometer is accurate. They have to be by law. I use a rough 8 -10% fudge factor based upon speed check radars when entering local towns and the flow of traffic.
If that is my biggest problem, I don't have any problems.
Monoflo, I also had a NA Miata for 6 years. Enjoyed it, but it was time to move on. I got a C280 MB after that and was amused to note it got the same gas mileage as the Miata and got me to my destination as quickly and in a lot more comfort.

Best
Les