So much waffle...
XCR, you should be fine with 215/65/16 (26.1" diameter). I ran 205/70/15 (26.3") and currently run 205/65/15 (25.5") on the rear without issue. I use
http://www.rimsntires.com/specspro.jsp for wheel/tyre sizing comparisons.
With these tyres and using the
DigiHUD app, when going 60 mph my speedo over reads by 1 mph.
It originally came with 205/55/16 on Auriga's like yours, which means the speedo under reads by 0.4 mph.
Sooo yeah, 940 speedos seem to be pretty damn accurate.
Here's a 760 on chunky 205/70/15's..
https://i.ebayimg.com/images/g/TO0AA...Wx/s-l1600.jpg
https://i.ebayimg.com/images/g/fMQAA...W9/s-l1600.jpg
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MY97 940 CD LPT Super Sports Edition 2 Turbo, Midnight Purple, 175,000 miles.