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"Safety Margin"
The topic of over optimistic speedometers has been discussed here several times. Some give a constant difference of 3 to 5 mph and others believe it is a 10 per cent difference. Either way, Porsche's answer is that it's a safety margin.
Not sure which model years this applies to, but on early models you can use the diagnostic settings on the HVAC display to accurate speeds in KPH. Yes, you can also find that little tidbit here on the forum. Enjoy!
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