I notice that when I use Garmin in my Boxster that there's a 4 mph difference. When the GPS shows 75 mph the cars speedo shows 71 mph. Always 4 mph...
I'm wondering which is accurate? Or....possible my 99 boxster might have come with standard 16's? I purchased it used with 17's. It is the "base" model. Maybe that would be the reason for the difference...if the math works?
