ScanGauge II Odometer off 3%, OEM Odometer off 1%

[vertrkr]

Since installing the ScanGauge II I've noticed after a few hundred miles it's trip odometer shows fewer miles traveled than the dash odometer by 4%. Meaning if the dash shows 200 miles the scangauge shows 192 miles.

I got out my Garmin 276c handheld GPS to see which is more accuarate. After a 100 mile test run I found the dash odometer is fast by 1% and the scangauge is slow by 3%:

GPS-100 miles
Dash-101 miles
S.G.-97 miles

Anyone else with a ScanGauge or GPS notice this? I contacted ScanGauge and they were baffled saying they've never seen more than a 1% error. I'm able to correct this in ScanGauge by bumping the Speed pref setting up by 3% so I get accurate odometer & mpg readings but of course the mph reading is fast now, not a big deal but a good head scratcher. Apparently the ODB II port doesn't give distance signals so ScanGauge calculates distance using Speed & Time. The odd thing is the gps, dash, scangauge all gave the same speed readings so perhaps the clock is off on the scangauge but I happen to have two scangauges and both gave the same readings, go figure.

[BillB]

How confident are you in the GPS being accurate? 1% off on the OEM odometer is a significant variance if this is an accurate measurement.

[ridged]

Originally Posted by BillB

How confident are you in the GPS being accurate? 1% off on the OEM odometer is a significant variance if this is an accurate measurement.

GPS that's available to civilians (outside of commercial entities such as land surveyors) is not 100% accurate. I don't know what the variance is but, as far as I know, it is close enough for everything but bomb placement.

geotech could likely enlighten us on all of the variables in the world of GPS location but from what I recall when I was in the surveying business, the whole concept is very dynamic.

[Me2]

Originally Posted by ridged

GPS that's available to civilians (outside of commercial entities such as land surveyors) is not 100% accurate. I don't know what the variance is but, as far as I know, it is close enough for everything but bomb placement.

geotech could likely enlighten us on all of the variables in the world of GPS location but from what I recall when I was in the surveying business, the whole concept is very dynamic.

As far as I know, they turned off that variance in accuracy in 2000. Apparently, they can jam the accurate signal locally if the need should arise, and can most likely turn it back on globally as well.

http://en.wikipedia.org/wiki/Gps#Selective_availability

[mugen1]

Highway markers, I guess.

[bonerep]

Originally Posted by vertrkr

Since installing the ScanGauge II I've noticed after a few hundred miles it's trip odometer shows fewer miles traveled than the dash odometer by 4%. Meaning if the dash shows 200 miles the scangauge shows 192 miles.

I got out my Garmin 276c handheld GPS to see which is more accuarate. After a 100 mile test run I found the dash odometer is fast by 1% and the scangauge is slow by 3%:

GPS-100 miles
Dash-101 miles
S.G.-97 miles

Anyone else with a ScanGauge or GPS notice this? I contacted ScanGauge and they were baffled saying they've never seen more than a 1% error. I'm able to correct this in ScanGauge by bumping the Speed pref setting up by 3% so I get accurate odometer & mpg readings but of course the mph reading is fast now, not a big deal but a good head scratcher. Apparently the ODB II port doesn't give distance signals so ScanGauge calculates distance using Speed & Time. The odd thing is the gps, dash, scangauge all gave the same speed readings so perhaps the clock is off on the scangauge but I happen to have two scangauges and both gave the same readings, go figure.

I am amazed that you have the time and will power to figure all that out....
Do you think the MPG is off too?

[MontanaFred]

I didn't need a ScanGauge to know my odometer was off close to 2 percent. I bought my Ridgeline in Texas and drove it back to Montana and I was using a directions print out from www.mapquest.com and I kept noticing that when the MapQuest directions said turn there was no turn - at least not for a few miles more. So I compared my odometer to the mile markers that are along the US highways and found that when my odometer says I've gone 61 miles I have really only gone 60 miles. When I bring it to the dealer for my second oil change I am going to ask them if they can adjust my odometer.

.

[djdj]

GPS distance measurement is only approximate. It doesn't elevation, terrain, and length of turns into account when making calculations. For a GPS distance reading to be right you'd have to travel on level ground in a straight line. If that was the case, the accuracy of GPS (+/- 10m) isn't much of a factor over a long distance. But the difference caused by terrain and turns introduces far more error into distance calculations than the the GPS signal itself.

Not that the odometer could really be relied upon either. Though the speedometer in the Ridgeline is the most accurate of any I have ever seen, so likely the odometer is probably more accurate as well. The reading is affected by tire pressure among other things.

The reading on the ScanGauge should be more accurate, though, especially since it is getting information from the RL. It sounds like it needs to be calibrated.

[swampler]

Originally Posted by djdj

GPS distance measurement is only approximate. It doesn't elevation, terrain, and length of turns into account when making calculations. For a GPS distance reading to be right you'd have to travel on level ground in a straight line. If that was the case, the accuracy of GPS (+/- 10m) isn't much of a factor over a long distance. But the difference caused by terrain and turns introduces far more error into distance calculations than the the GPS signal itself.

You sure about that? My GPS shows elevation, which makes it logical that elevation would be taken into account, otherwise it would be worthless.

[bongus]

My in dash Navi does not have elevation but it denotes slope...would this suffice?