Thanks Chris.

Good to hear I am not completly lost
The TB15S 270/45VR15 have 625 mm diameter, so that means around + 8% different to your tires but only - 1,5% off from the Original Alfa 195/65 x 15".
So should be 1,5 % more exact than the stock Alfa then. And there are always a built in error on the safe side of the speedlimit, that is probably why you get a accurate reading.

In Norway we drive by the speedlimit + VAT, witch is 25% and usually that is fine with the built in errors of the speedo + error at the police radar.

My concern was that the ratios inside the speedometer of the original Alfa and the original Stratos stradale, Dino or 246 GT speedometer was not the same.

What kind of speedo do you have in your car?

Regards

René