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Old 06-07-2007, 10:37 AM   #1
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Question: Speedometer vs. Odometer

I would have to agree with the posters wo report that the speedometer reads higher than actual speed (I forget the %) My question is whether the odometer is recording actual miles or does it record high as well? Thanks.

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Old 06-07-2007, 11:27 AM   #2
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I think the speedometer is separate from the odometer. I have one of those police speed signs by my house and I average +2-3. So when I am going 43MPH it says I am going 40MPH. But that could just be my car or the speed meter being off. No way to really now without taking it into a shop. My sister has a Mini and always tells me I go 5 under.
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Old 06-07-2007, 11:39 AM   #3
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Quote:
Originally Posted by rick3000
I think the speedometer is separate from the odometer. I have one of those police speed signs by my house and I average +2-3. So when I am going 43MPH it says I am going 40MPH. But that could just be my car or the speed meter being off. No way to really now without taking it into a shop. My sister has a Mini and always tells me I go 5 under.
I would not go by the police speed sign meter. Those things need to be re-calibrated every so often. Same as the cops radar/laser guns they use. They can be off by a few mph.

My speedometer and the digital read-out are in sink, but weather they are both off from true mph is another story. It could have allot to do with your wheel and tire set up too. Depends on how extreem you went with a plus size, like a set of 19s. Could make your readings off a little from the actual speed showing.
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Old 06-07-2007, 02:49 PM   #4
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I have come to the conclusion my speedometer is off by 3mph also. Curiously it doesn't seem to be a percentage based on the speed but appears to be a constant 3mph across the board. This is based on the speed signs, but from different cities and states. Would they all be wrong by exactly 3pmh? Seems too coincidental. Except for the set of tires I currently have, the previous sets were to factory specs. I have not checked speed with the new set I have on now which are a size or two larger than stock but still 18's. I also wondered if the odometer was registering more miles based on the "higher" speed, so I checked using mile markers on an Interstate. I went 10 miles and the odometer said exactly 10 miles to the tenth.

Is there any way to correct the speedometer?
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Old 06-07-2007, 03:19 PM   #5
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I checked my indicated speed with my GPS when I was coming back from Phoenix.

At 80mph as indicated by the speedo, GPS says I was doing 76mph. A little faster, and the difference was about 6mph.

It's been rumored before that Porsche sets it's speedometers to indicate that you are going a little faster than you actually are.
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Old 06-07-2007, 03:54 PM   #6
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Mine is off by 3 mph too.
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Old 06-07-2007, 04:04 PM   #7
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Maybe porsche sets them 3 mph low so that porsche owners don't get a reputation for speeding tickets. So when you go 8mph over your really only going 5mph over and a cop with over look 5mph.
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Old 06-07-2007, 04:09 PM   #8
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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.
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Old 06-07-2007, 06:49 PM   #9
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Go out on the interstate and time yourself between mile markers at some constant speed, 60 is convenient. Divide the time in seconds into 3600.
If it takes you for example 62 seconds to go a mile instead of 60 seconds, your actual average speed over that mile is 58mph.
Jeph's discussion of odo error is why rallies usually have an odo check section, so that the individual car odo is compared with the device (odometer) that measured the rally. Variations of 10 to 20% are not uncommon.

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Old 06-07-2007, 07:05 PM   #10
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Your digital readout will be 2,3 or even 4+ MPH HIGHER than the analog readout at certain speeds. Compare them and you will notice that 40 MPH on the analog is showing 42-43 on the digital. I checked this against my GPS and confirmed that the digital is showing a higher speed.
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Old 06-07-2007, 08:44 PM   #11
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I have checked both with my GPS, the speedo on mine is off by 4 mph, but the odometer is spot on.
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Old 06-08-2007, 12:06 AM   #12
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I read recently where a certain Japaneese maunufacturer was being sued via class action because their odometer readings were reading higher than the federal codes allowed( I think it was plus or minus 5%). The new car owners were complaining that they were being cheated out of their full warranty coverage. The manufacturer agreed to give them credit for the overcharged mileage...
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Old 06-08-2007, 06:31 AM   #13
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I've noticed mine is about 3mph off as well. I base this on the GPS I have in the car.

3mph isn't bad at all. In the sportbike world your speed may be off by 10% or more than indicated. That's on a stock bike, too.

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