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Discussion Starter #1
If the speedometer shows 70 mph but I'm reality you are only going 67mph:

1. Is it merely trying to protect you from getting a ticket?

2. Or indeed is it piling up the miles over and above what should rightly be registered on the odometer?

Reason I ask: thanks for asking.

Waze, which uses GPS which I would think would be very accurate, shows me going 3 mph slower than the speedometer.

What gives?

Now this is nothing to sneeze at. There was a successful class action suit against an automaker (why does VW come to mind, but my memory may be faulty) where it was piling up miles inaccurately.
 

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Discussion Starter #3
Now that was very informative.

BUT: still an unanswered question. Is the odometer thus as inaccurate as the speedo?
 

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I think this is pretty normal in P cars as well as BMW's.
 

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There are only 2 possibilities here.

1. Porsche and some other car makers cannot make an accurate speedo = BS

2. Porsche and some car makers intentionally make the speedo faster than the actual speed to protect you/cover their ass = bigger BS

Any car I have driven never showed a discrepancy of more than 1-2 mph (normal variation, I believe). I have never heard of any car maker being sued for speedo inaccuracy resulting in driver getting a ticket/in an accident.
 

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I think the OPs original question revolved around the speedo AND odometer, ie is the mileage racking up faster than reality. I too have questioned this as a 3mph discrepancy in the speedo at 60 equates to 5 percent. If we assume speedo diff translates to mileage diff (which I do not believe is the case), then warranties run out 5% earlier, than they should which could arguably cut Porsches warranty costs by 5% assuming a bunch of things like constant rate of breakage etc.
 

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Discussion Starter #8
@Rush

Thank you. That is exactly my point. But also for those who lease, are their miles building up faster? Are they paying and getting 5 percent less miles? There are many smart people around here, answer the question.
 

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I do recall a similar post and someone had checked their odometer against roadside markers. I don't recall the distance but the result was accurate based on highway mileage posts. Not the X miles to nowhere signs, but rather the mileage check posts.
 

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My speedo also reads 3 mph fast. This is comparing speedo reading to roadside "your speed is" radar signs which then usually flash "slow down"!
 
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I think the OPs original question revolved around the speedo AND odometer, ie is the mileage racking up faster than reality.
that was not the OP's question. The question was "the speedometer shows 70 mph but I'm reality you are only going 67mph:" There was nothing about the odometer.

If the OP read the reference article, the answer was there. It even specifically mentions Porsches.

This is well known that the German speedos are optimistic. Be thankful. It might save a ticket. If you had read it, it says "The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low."

If you don't believe any of this, read this Why you may not be driving as fast as you think - The Globe and Mail the decade old C&D article remains the standard explanation.

So there you go. EU regulations mandate a pessimistic speedo.

But the ODO has its own sensors and all is fine.

No worries, all is cool. Yes, Google really is your friend ;)
 

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Discussion Starter #12
@grim


Read my original post a bit better, sir, and you will see my final sentence referred to miles piling up. Is that not a mention of the odometer?


And I did read the article you linked and said as much in my next post after yours. And unless I missed it, as you missed what I wrote, I saw nothing about the odometer being accurate in spite of the speedo not.


In addition while it may be "old" news to you, not everyone is as knowledgeable or has read everything there is to read. And are you suggesting that before we post any topic, we make sure the answer doesn't lie somewhere in cyberspace?


Take a chill pill and a good look at your screen name, grim.


In your post about purchasing cars, someone commented regarding your replies often being cutting and somewhat rude. You acknowledged same and said you were working on that. May I be so bold as to suggest you work harder.
 

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The speedometer and the odometer are mutually exclusive. You can check other resources to confirm this but basically the odometer counts revolutions of the wheel and is extremely accurate (or technically can be). However the way speed works as revolutions per time it has to be calibrated at a certain speed. Departures from this calibrated speed have a standard deviation of error. Way beyond my technical ability to explain but you can research same. Long story short your odometer is accurate at any speed but your speedometer will have a variance. The fact that you read 70 vs 67 does not mean your odometer is inaccurate. Hope this helps and if you are interested there are excellent articles on the internet that go into great depths to explain same.
 

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@grim


Read my original post a bit better, sir, and you will see my final sentence referred to miles piling up. Is that not a mention of the odometer?


And I did read the article you linked and said as much in my next post after yours. And unless I missed it, as you missed what I wrote, I saw nothing about the odometer being accurate in spite of the speedo not.


In addition while it may be "old" news to you, not everyone is as knowledgeable or has read everything there is to read. And are you suggesting that before we post any topic, we make sure the answer doesn't lie somewhere in cyberspace?


Take a chill pill and a good look at your screen name, grim.


In your post about purchasing cars, someone commented regarding your replies often being cutting and somewhat rude. You acknowledged same and said you were working on that. May I be so bold as to suggest you work harder.
Your odo question was not a question but a statement buried at the end, so yes, I took that as a statement, not a question. If you had asked that up front, then it would have been more obvious. The second reference explicitly explained the speedo and odo are not linked.

No, you should ask anything you want. This question was asked before here and discussed. It will also be asked again in 6 months, etc. That's cool. Ask away.
 

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I had an advisor confess to me that he had just made an undergraduate cry in his office. She was struggling with a problem and he popped off "I've told you guys a thousand times that the way to solve...." She broke down and he realized, "yeah, I've said it a thousand times, but she's not heard it before". Something like that.

I agree, I've even been quick to punch the "search the forum response" button. Old timers (not disrespectfully of course) have seen most of the questions and answers already and we learn a lot from them. Sometimes they get the opportunity to teach the same thing twice, or even thrice.
 

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The GPS speed on my Escort Max is routinely 2-3 MPH lower than the car's speedo reading.
 
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