accuracy of GPS mph compared to speedometer
#11
06-25-2007, 06:16 PM
Re: accuracy of GPS mph compared to speedometer
To answer your questions about GPS heading:
1. you have to be moving for GPS to show any direction, if the car is parked there is no way for GPS to know where the car is facing.
2. There are actually 2 types of North: magnetic and true North. As the name would suggest Magnetic North would be North as shown on magnetic compass and point to somewhere in North Canada at the moment (it slowly changes) and it's different for different parts of US. The difference between true North (pointing straight to the North Pole) and magnetic North is called declination and for continental US can be between -20 to +20 degrees depending on location.
As you probably already guessed, Compass will always show magnetic North.
With GPS it is preferable to use true North, however magnetic North could be calculated using correction tables as long as you know your location. I would imagine your discrepancy is due to one showing magnetic North and the other showing true North unless your electronic compass is not calibrated and adjusted especially inside all metal car. I hope this helps.
1. you have to be moving for GPS to show any direction, if the car is parked there is no way for GPS to know where the car is facing.
2. There are actually 2 types of North: magnetic and true North. As the name would suggest Magnetic North would be North as shown on magnetic compass and point to somewhere in North Canada at the moment (it slowly changes) and it's different for different parts of US. The difference between true North (pointing straight to the North Pole) and magnetic North is called declination and for continental US can be between -20 to +20 degrees depending on location.
As you probably already guessed, Compass will always show magnetic North.
With GPS it is preferable to use true North, however magnetic North could be calculated using correction tables as long as you know your location. I would imagine your discrepancy is due to one showing magnetic North and the other showing true North unless your electronic compass is not calibrated and adjusted especially inside all metal car. I hope this helps.
#14
06-26-2007, 02:38 PM
Re: accuracy of GPS mph compared to speedometer
Thank you all for your responce. I also have a garmin gps and it is dead on with the car I am driving now. I believe the garmin is very accurate on speed. I get gps with the 50th anniversary camry, but will keep the garmin in the family. One more thing, does the camry gps show the mph all the time or do you have to bring up a particular screen to get it. Thanx..........
#15
06-26-2007, 02:54 PM
Re: accuracy of GPS mph compared to speedometer
Camry gps does not show speed. At least I haven't found it if it does. There are some very nice features with the Camry gps but you will also find yourself wishing it worked a little like the garmin.
#17
06-27-2007, 12:22 PM
Re: accuracy of GPS mph compared to speedometer
The GPS display can show a speed sensor reading that shows speed, but only when you are in a special diagnostics screen. I don't think this is coming from GPS signals, though. It appears to be showing the reading from a rotation sensor in the transaxle (measuring rotation of MG2)... This speed value varies by about 4mph from my speedo reading at 75 mph.
See:
https://www.greenhybrid.com/discuss/...ad.php?t=13123
and
https://www.greenhybrid.com/discuss/...ad.php?t=11376
See:
https://www.greenhybrid.com/discuss/...ad.php?t=13123
and
https://www.greenhybrid.com/discuss/...ad.php?t=11376
#18
07-04-2007, 10:49 AM
Re: accuracy of GPS mph compared to speedometer
Have a look at the attached pages ME-39to42.pdf from the TCH Repair Manual. They give the accuracy tolerances of the speedometer (among other things). I believe that US and international regulations now require that the speedometer should not under-read, hence the tolerances are such as to make it over-read. In my case (metric Canadian TCH), my speedometer is about 1% high compared with a GPS speed reading [on a straight road, that is; if the road curves, the GPS will read low because it computes along the chord which is shorter than the actual curved path]. To get my ScanGauge to agree with the car's speedometer, I need to set its "speed" calibration to +1 or +2%. [This also makes ScanGauge's trip distance measurements agree within ~0.5% with the car's odometer.]
Stan
Stan
#19
06-27-2009, 05:40 AM
Re: accuracy of GPS mph compared to speedometer
My new 09 Camry Hybrid is also off 3mph.... If the speedometer is showing more than it is really travelling, won't that make the mileage and gas usage "off" as well? Seems like Toyota could do this on purpose so they could market this as getting more mpg than it does. It is a shame to buy a new car that does not have equipment that works properly.
#20
06-27-2009, 08:09 AM
Re: accuracy of GPS mph compared to speedometer
I think they err on the speed for a few reasons..
they want people to drive a bit slower.
there is less likelihood of a lawsuit if the speedometer caused someone to inadvertently speed due to an error (if the car registered a slower speed than it was traveling)
I don't believe the advertised MPG is related to the onboard computers, rather it might be based upon EPA standards
All these are my opinions...
don't worry about it...it's only a car, just drive it and enjoy it.
they want people to drive a bit slower.
there is less likelihood of a lawsuit if the speedometer caused someone to inadvertently speed due to an error (if the car registered a slower speed than it was traveling)
I don't believe the advertised MPG is related to the onboard computers, rather it might be based upon EPA standards
All these are my opinions...
don't worry about it...it's only a car, just drive it and enjoy it.
