[Brian in Tucson]

Your digital speedometer will still be in mph, as will your trip meter and odometer will still be in miles. And there's a chance you will fubar the needle or needle drive. 0 position is where you should start at, gently try to pull the needle off. After you're done, check the accuracy with gps, you can adjust it by pulling the needle back off and repositioning it.

Be gentle!

I suspect you're in Europe. If all else fails, you can probably find a used cluster on Ebay.

Note, I've never done this on a Porsche, but have done it on American cars to adjust the accuracy of the speedometer.