inaccurate speedometer

[Bob O]

I too have the exact same readings. GPS says 70, speedo says 74. The difference varies from roughly 3-5% depending on speed. Don't think it has anything to do with tires, at least standard size tires. My 16" snows and summer 17's give the same error at the same speed. Somewhere a while back I read that Porsche deliberately overstates the speed for safety and because there's some sort of huge penalty to the mfg if the speedo's read slow. (Can you believe anything you read on the internet?) I certainly don't know that for sure but this issue does seem to be common.

Oddly, when I was at Boxstoberfest last week, I was talking with a guy who had an 07 S. We were following him from Dallas to FB and when we stopped for a quick break several of us were comparing what our speedos were reading. He said he had just compared his speedo with his GPS and it was dead on accurate (or at least the two agreed). Maybe Porsche more accurately calibrates the speedos in the newer models???

Bob