Odometer off by 3%
03-19-2009, 02:25 PM
#1
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Odometer off by 3%
I have noticed since my xD was new that the speedo is off a bit. At 59 MPG on the gauge, I'm going 60 according to GPS. Anyway, I wondered if the odometer was off, too... so this morning I zeroed my GPS and the Trip B meter at the same time. After driving 25.7 miles according to the Trip B, the GPS was reading 26.5. So, that means my xD's odometer reads about 3% low.
To put that in slightly larger numbers, when I went 380 miles on a tank a few weeks ago, I had actually gone 391 miles. When I calculated my fuel economy at 38.1 MPG, it was actually closer to 39.2 MPG. And now that my car thinks it has 8,000 miles on it, it really has 8,240.
I have stock 16" alloys with stock tires that are properly inflated (well, a little over the Scion recommendation).
Just wanted to put this info out there in case anyone is interested. The good thing is that you guys/gals who are calculating your FE at the pump are probably getting slightly better MPG than you think! Not as good as the MPG display, though... which tends to read 7-8% high.
To put that in slightly larger numbers, when I went 380 miles on a tank a few weeks ago, I had actually gone 391 miles. When I calculated my fuel economy at 38.1 MPG, it was actually closer to 39.2 MPG. And now that my car thinks it has 8,000 miles on it, it really has 8,240.
I have stock 16" alloys with stock tires that are properly inflated (well, a little over the Scion recommendation).
Just wanted to put this info out there in case anyone is interested. The good thing is that you guys/gals who are calculating your FE at the pump are probably getting slightly better MPG than you think! Not as good as the MPG display, though... which tends to read 7-8% high.
03-19-2009, 03:26 PM
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Scikotics
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Good info McBrew,
Now you know the only way to tell if your GPS is ***** on accurate is to walk off a distance like a mile and measure it with all the available devices you have to see which is actually off.
But at least it sounds like our cars would be off to our benefit as far as warranty mileage goes .. LOL
Now you know the only way to tell if your GPS is ***** on accurate is to walk off a distance like a mile and measure it with all the available devices you have to see which is actually off.
But at least it sounds like our cars would be off to our benefit as far as warranty mileage goes .. LOL
03-19-2009, 05:09 PM
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Originally Posted by TheQuietThings
I would put my money on the GPS
Also, this trip was all open areas. No dense foliage, no tall buildings, no tunnels, etc. Next time I head up to PA I'll do the same thing. That'll be just over 100 miles, so I'll get an even better idea of the error percentage.
It's funny... the xD speedo reads a little low and the odometer reads even lower. My Golf speedo read high and the odometer was less than 0.5% off.
03-19-2009, 08:16 PM
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maybe it has something to do with turning? the gps can measure the actual distance it takes you to make a turn, but the car cant compensate for the change in wheel speed when you are turning? i dunno, i guess it depends on where the speedo takes its measurement from.
03-20-2009, 02:48 PM
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Chels, SG can only tell you what the engine computer is reporting. I'm assuming that the odometer is not a "dummy gauge," so it is going to report the same numbers as the scangauge would. In fact, I'm pretty sure it has to, by law.
It would be almost impossible for any odometer to be exactly correct. I was just a little surprised by how far off it was.
Also, I doubt the turns could account for so much. I can see it having an impact... but not a difference of more than a mile out of every 34.
GPS, so long as the signal is uninterrupted, should be very accurate. Not so much for VERY short distances, like 50 feet, but for miles and miles, they should be more accurate than the simple odometer on a car.
It would be almost impossible for any odometer to be exactly correct. I was just a little surprised by how far off it was.
Also, I doubt the turns could account for so much. I can see it having an impact... but not a difference of more than a mile out of every 34.
GPS, so long as the signal is uninterrupted, should be very accurate. Not so much for VERY short distances, like 50 feet, but for miles and miles, they should be more accurate than the simple odometer on a car.
03-20-2009, 07:28 PM
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I know you guys swear by these gps units, but bear this in mind.
The commercially available ones we have are not meant to be dead on ***** accurate. I know this because I read a paper where a professor used one to see how accurately they could plot paths for spraying for locusts in a desert.
His tests concluded that even when standing still, the signal degradation left a margin of error of over 100m x 100m. Which meant on his graph it looked like the gps was moving everywhere when it was actually still.
That lead to a second test where they did a straight line 500m distance test, the end result had the gps reading 838m, thats a big deal.
So all I am saying is don't hold your equipment as gospel. If it is consistently off it still gives a baseline for any type of measurement.
And no he did not mention brands as this was not a product test but rather helped reach a conclusion as to why the locust problem he was having had not been resolved ... they were spraying in the wrong area ... LOL
The commercially available ones we have are not meant to be dead on ***** accurate. I know this because I read a paper where a professor used one to see how accurately they could plot paths for spraying for locusts in a desert.
His tests concluded that even when standing still, the signal degradation left a margin of error of over 100m x 100m. Which meant on his graph it looked like the gps was moving everywhere when it was actually still.
That lead to a second test where they did a straight line 500m distance test, the end result had the gps reading 838m, thats a big deal.
So all I am saying is don't hold your equipment as gospel. If it is consistently off it still gives a baseline for any type of measurement.
And no he did not mention brands as this was not a product test but rather helped reach a conclusion as to why the locust problem he was having had not been resolved ... they were spraying in the wrong area ... LOL
03-21-2009, 05:27 AM
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Originally Posted by trikkonceptz
I know you guys swear by these gps units, but bear this in mind.
The commercially available ones we have are not meant to be dead on ***** accurate. I know this because I read a paper where a professor used one to see how accurately they could plot paths for spraying for locusts in a desert.
His tests concluded that even when standing still, the signal degradation left a margin of error of over 100m x 100m. Which meant on his graph it looked like the gps was moving everywhere when it was actually still.
That lead to a second test where they did a straight line 500m distance test, the end result had the gps reading 838m, thats a big deal.
So all I am saying is don't hold your equipment as gospel. If it is consistently off it still gives a baseline for any type of measurement.
The commercially available ones we have are not meant to be dead on ***** accurate. I know this because I read a paper where a professor used one to see how accurately they could plot paths for spraying for locusts in a desert.
His tests concluded that even when standing still, the signal degradation left a margin of error of over 100m x 100m. Which meant on his graph it looked like the gps was moving everywhere when it was actually still.
That lead to a second test where they did a straight line 500m distance test, the end result had the gps reading 838m, thats a big deal.
So all I am saying is don't hold your equipment as gospel. If it is consistently off it still gives a baseline for any type of measurement.
03-21-2009, 01:10 PM
#12
my verizon navigator, when stopped, still moves by "seconds" by latitude and longitude settings.
like "degrees, minutes, seconds" even though those seconds are only feet apart, it still shows me moving. in a circle, but still moving. LOL
like "degrees, minutes, seconds" even though those seconds are only feet apart, it still shows me moving. in a circle, but still moving. LOL
03-22-2009, 02:01 AM
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Stock 16" wheels and tires.
I have another GPS unit and will test it with that one this week.
Seriously, though... it is NOT unusual at all for an odometer to be a few percent off. My VW Golf was the closest I ever had, and that was less than a percent off. I don't think 3% is all that bad... I'm just glad it is off the way it is. If it was 3% off the other way, my fuel economy would be worse than what I think it is.
I have another GPS unit and will test it with that one this week.
Seriously, though... it is NOT unusual at all for an odometer to be a few percent off. My VW Golf was the closest I ever had, and that was less than a percent off. I don't think 3% is all that bad... I'm just glad it is off the way it is. If it was 3% off the other way, my fuel economy would be worse than what I think it is.
03-26-2009, 02:51 PM
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All I have to say is that my GPS thinks my house is 70 yard further down the road when I am parked in my driveway...
oh and IMHO computers are perfect, they do exactly what they are programmed to do... now if the programming is bad/not accurate, thats a different story... that would be the human error from the original programmer...
oh and IMHO computers are perfect, they do exactly what they are programmed to do... now if the programming is bad/not accurate, thats a different story... that would be the human error from the original programmer...
03-26-2009, 04:01 PM
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If your GPS thinks the house is in a different spot, it is because the map in your GPS is not 100% correct. In fact, none of the maps know where any houses are. They are supposed to know where the property is... but that's often incorrect, too. I have a feeling that when they make the maps, they divide the street evenly. In other words, if the first property on the street is address 10 and the last one on the street is address 90, then the map assumes that 50 is right in the middle of the street. Of course, it is not always going to work out that way.
03-26-2009, 06:45 PM
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I am a speedometer and odometer repair man ( hence my screen name )
I must say, I do accuracy checks all day long, and a 3% error is very small
Most auto manufacturers tend to set odometer calibration 7% fast on purpose, the sae standard for an odometer states that it must read within 7.1%, and auto makers usually want it to read as high as possible ( causing you to run out of warranty faster )
Just a little info I thought I would share
I must say, I do accuracy checks all day long, and a 3% error is very small
Most auto manufacturers tend to set odometer calibration 7% fast on purpose, the sae standard for an odometer states that it must read within 7.1%, and auto makers usually want it to read as high as possible ( causing you to run out of warranty faster )
Just a little info I thought I would share
03-26-2009, 07:01 PM
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Originally Posted by speedoguy
I am a speedometer and odometer repair man ( hence my screen name )
I must say, I do accuracy checks all day long, and a 3% error is very small
Most auto manufacturers tend to set odometer calibration 7% fast on purpose, the sae standard for an odometer states that it must read within 7.1%, and auto makers usually want it to read as high as possible ( causing you to run out of warranty faster )
Just a little info I thought I would share
I must say, I do accuracy checks all day long, and a 3% error is very small
Most auto manufacturers tend to set odometer calibration 7% fast on purpose, the sae standard for an odometer states that it must read within 7.1%, and auto makers usually want it to read as high as possible ( causing you to run out of warranty faster )
Just a little info I thought I would share
how can we tell if our speedo is accurate?
and what do you need to do in order to recalibrate it?