thoughts on tire diameter and speedo correction

[Jinster]

I know that a lot of Boxster's speedo indicate about 10% faster than actual speed. Would a simple increase in tire wall length correct the situation? I am not sure whether bigger tires would fit though. Also, there would be concerns regarding why Porsche doesn't approve 18 inch rims on 1997 models and whether an increase in tire diameter would result in the same problem that caused Porsche to disapprove 18 inch wheels.

Does anyone have any thoughts?

[adsach]

Hoping this isn't a double post

I remember someone (Trygve) measuring and graphing and coming up with an error rate of -5% + -2MPH(3KPH).

Here is a link to a tire size calculator page

http://www.1010tires.com/TireSizeCalculato...p?action=submit

Here is a link to a PPBB post explaing why you might not care

http://www.ppbb.com/board/museum/te-marer3.htm

Personally, I would consider tire sizes like 245/40/18 in front and 285/35/18 in the rear for reasons of added traction rather than as an attempt to correct a speedometer error.