Finally Fulltiming
Summer: Finger Lakes, NY; Winter: Somewhere warm
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Apologies if this has been covered - couldn't find anything.
Had a TomTom GPS & it consistently indicated that my speedometer was 2 mph low at all speeds. Recently replaced with a Garmin (also got 2 new front tires, same size) & it indicates about a 1 mph difference.
I've checked the speedometer against roadside signs many times & it consistently shows about .1 miles more than actual in a 10 mile distance. I guess this is inconsequential, considering a slight wavering over that length of time?
Also heard or read a comment here once that "all vehicle speedometers are set 2 mph fast".
Anyone confirm that statement or have similar experiences?
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Polishnurse
Schodack, NY
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I have several different GPS units and use them all in different Vehicle at different times. They all give me different speeds as to the Odometer in the vehicle. The Jeep is always 4 MPH under the GPS or more depending on the GPS.
I tend to believe the GPS only in that tire size and the amount of air will cause this variance. JM2Cents Bill
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Briteskys
Mesa, AZ
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One would have to understand all aspects of GPS triangulation/earth plane surface algorithms, etc to say that one or the other is more accurate across all elevations. Then there are tire variations, road variations, and other factors ad nauseum.
If you are within a couple mph's why sweat the small stuff. 
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FXSTOHIO
Ohio
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I read an article one time about Toyota or Honda don’t remember which one it was but their speedometer was fast by about 5 miles per hour and people were complaining about not getting the correct mileage on their odometers.
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jay2003
San Joaquin Valley, California
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Speedo's are allowed to be up to 2% off.. and as was mentioned.. tire pressure and other factors change the MPH reading. GPS can also be off but up to 1% depending on the number of sats being tracked and the angle they are in relationship to your unit.
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MrWizard
Van Nuys, Ca
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could it be 'terrain' GPS measures geographical straight miles ? it measures map co-ordiantes, not miles driven up & down hills and curves & twisty roads.
any calculations performed by the gps, time vs distance are going to use the measured point-to-point distance and compare it to its mapped data base, the point to point will almost always be shorter than the driven distance, (except for straight lines on city streets) and the map database might not take into account terrain changes
assuming the data base uses 'surveyors' measurements. in the data base, when was the last time the route was surveyed.
ME i wouldn't worry about the speed differences, the only thing about a gps that interests me is 'my location'.
hold 60 mph for that 10miles, it takes 10 minutes, if it takes less time your speedo reads slow, if it takes longer your speedo reads fast and the gps is correct
correct 60mph =1mile per minute 10miles=10minutes
* This post was
edited 10/07/09 07:47pm by MrWizard *
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Finally Fulltiming
Summer: Finger Lakes, NY; Winter: Somewhere warm
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Thanks for the replies, guys. Pretty much what I figured & I don't really consider it a major problem - just wondered if others had the same experiences.
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Grillmeister
Iconoclast
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Quote:hold 60 mph for that 10miles, it takes 6 minutes You meant 10 minutes right?
Show me the GRILL and STAND BACK!!!! 
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fordsooperdooty
Orange Kounty Kalifornia
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My manual calculations and my Scan Guage both indicate my speedometer is around 3-4 MPH off my trucks speedometer at 65 mph. The manufacturers factory setting for vehicle calibration for speedometers accuracy is mandated federally to + or - 10%. The GPS navigators in civilian use are accurate as far as speed is concerned, and when the more satellites are locked in generally are + or - 2 mph average.
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Kamphiker
South Florida (this 'aint paradise anymore)
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And now you know why there variations in gas millage (MPG) everyone claims.
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