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Old 08-26-2008, 02:35 PM   #1
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Odometer/Speedometer off

'99-986 Noticed while on a long trip that the odometer and speedometer are off -consistently high by 7% or so when checked against interstate mileage markers. Any ideas? New tires added - same size as old ones.

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Old 08-26-2008, 02:56 PM   #2
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Check against a GPS, not highway mileage markers. That would be more accurate.

What are your tire sizes?
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Old 08-26-2008, 03:05 PM   #3
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check against gps

That was the first clue. Speed on the GPS was about 7% lower than speedometer. Checked against mile markers over a 50 mile stretch of interstate and got about the same answer.

Don't remember tire size off the top of my head and car is in shop for maintenance and to look at the issue.
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Old 08-26-2008, 03:40 PM   #4
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This is entirely 'normal'. Buddy had a 99 996, and it was the same. My 2001 is the same. Heck, my motorcycle is the same.

I believe it probably has something to do with the calibration erring on the side of caution. You are less likely to get speeding citations if you're actually going slower than you think you are.

It also limits liability of the manufacturer. Can you imagine if your speedo indicated SLOWER than you were going? By 5-10mph at highway speeds? That's the difference between a speeding ticket or not.

Anyway. That's the way it is. There's nothing 'wrong' with your car.
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Old 08-26-2008, 05:32 PM   #5
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I love my Boxster, but I personally find this characteristic the most annoying thing about the car. I hate merging onto the highway doing what I think is a couple of mph over the speed limit and the next you know I am getting passed by Chevy Vegas and Ford Pintos like I am standing still. I have even had teeny boppers yell at me. "Won't that thing go any faster!" It's embarrassing.
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Old 08-26-2008, 07:49 PM   #6
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Porsche calls it a safety factor and it (speedo) averages 5mph over actual speed.

But the odo and the speedo get their info from different sources and the odo is usually spot on. You may have other issues, as stated, tire size or such.
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Old 08-27-2008, 06:10 AM   #7
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Thanks everyone...I'll relax

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