accuracy of GPS miles per hour compared to speedometer - Page 2 - GreenHybrid

spiff72 · #11 (**permalink**) 06-25-2007, 06:16 PM

Quote:

Originally Posted by Pete4

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.

Actually I have observed this diagnostic screen several additional times since I originally posted, and I think I have found the answer to my own question. This "relative bearing" shown when entering the diagnostic screen appears to be relative to your heading when you ENTERED diagnostic mode. I should have made clearer the fact that the the relative bearing will show values of 0 degrees when you are heading due west or due south. This is because I was heading either due west or due south when I entered the GPS diagnostic screen.

Mudflap · #12 (**permalink**) 06-26-2007, 11:40 AM

My Garmin GPS is 2 miles per hour slower than what the speedo is saying at most speeds.

abward · #13 (**permalink**) 06-26-2007, 11:52 AM

Quote:

Originally Posted by Mudflap

My Garmin GPS is 2 miles per hour slower than what the speedo is saying at most speeds.

Mine too.

acco20 · #14 (**permalink**) 06-26-2007, 02:38 PM

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 miles per hour all the time or do you have to bring up a particular screen to get it. Thanx..........

Mudflap · #15 (**permalink**) 06-26-2007, 02:54 PM

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.

voodoo · #16 (**permalink**) 06-27-2007, 07:44 AM

ecco20 check your private message file for a letter from me. voodoo

spiff72 · #17 (**permalink**) 06-27-2007, 12:22 PM

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 miles per hour.

See:

http://www.greenhybrid.com/discuss/s...ad.php?t=13123

and

http://www.greenhybrid.com/discuss/s...ad.php?t=11376

SPL · #18 (**permalink**) 07-04-2007, 10:49 AM

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