[Jeph]

Too answer the second part of you question, I don't think the MPH directly related to the odometer. That would mean every Boxster (possible Porsche) owner would be getting the shaft, having the odometers indicating way more miles than actual miles. Lawsuits would go flying I'd think.

That's a really good question though, and I never really thought about that. You could do some testing though to find out for sure... If you have one mile marked, see how it lines up with the odometer...

...Or if you if you drive for 10 minutes at 60mph consistently, will you cover 10 miles? If the speedometer is off by 3mph at 60mph, then you will be 1-2 miles short for 20 miles. Confirm with either landmarks of a GPS and compare that to the odometer.

I would predict you would have to go 63mph for 10 minutes to travel exactly 10 miles.