accuracy of GPS mph compared to speedometer
#1
06-24-2007, 03:10 PM
accuracy of GPS mph compared to speedometer
Am waiting patiently for my new 50th anniversary edition camry hybrid. I have read somewhere that the speedometer is off, I think it was 1 or 2 miles per hour fast. Do any of you folks with the gps find this to be true, and is the gps mph reading accurate.
#2
06-24-2007, 04:07 PM
Re: accuracy of GPS mph compared to speedometer
I have yet to check the speedo against GPS BUT, just yesterday I went past one of those automated radar trailers that various PD's set out on the side of the road, you know the ones that say "Your Speed" along with a placard of the posted speed.
I found my speedo to be about 2.5 to 3 mph under the radar calculation. This is entirely consistent with what others have reported on this board.
Anyone with a 9th grade education in geometry and algebra could design an accurate speedometer. I have to believe Toyota did this on purpose. But why?
I found my speedo to be about 2.5 to 3 mph under the radar calculation. This is entirely consistent with what others have reported on this board.
Anyone with a 9th grade education in geometry and algebra could design an accurate speedometer. I have to believe Toyota did this on purpose. But why?
#3
06-24-2007, 04:19 PM
Re: accuracy of GPS mph compared to speedometer
acco20: I checked my wife's TCH against a Magellan 315 GPS last fall and found that the GPS read 67 mph and the speedometer on the car read 70 mph.
Not sure how accurate the GPS is.
Not sure how accurate the GPS is.
Last edited by David Price; 06-24-2007 at 04:25 PM.
#4
06-24-2007, 04:28 PM
Re: accuracy of GPS mph compared to speedometer
With SA (selective availability) turned off back in 1998, if you've got a minimum 3 bird lock, your GPS will be scary-accurate with speed. Position might be another story.
#5
06-24-2007, 04:49 PM
Re: accuracy of GPS mph compared to speedometer
This is a frequently discussed topic, going back to last spring when the first TCH's arrived. The general consensus is that the speedos are about 2-3 mph "fast." Whether Toyota did this on purpose is a matter of conjecture. Others have reported that various other models, by other manufacturers have speedos of varying accuracy.
You learn to adjust. I now know that I have a few mph's of leeway when I become concerned about the speed limit on a particular road.
#6
06-24-2007, 06:22 PM
Re: accuracy of GPS mph compared to speedometer
By the way; the odometer read 61 miles and the GPS read 61.9 miles.
#7
06-24-2007, 07:36 PM
Re: accuracy of GPS mph compared to speedometer
Check out this post from last year:
https://www.greenhybrid.com/discuss/...ad.php?t=10105
Basically, the speedometer was found to be about 3mph off at 60mph, which caused a 1.5% mileage error when compared to a GPS system.
https://www.greenhybrid.com/discuss/...ad.php?t=10105
Basically, the speedometer was found to be about 3mph off at 60mph, which caused a 1.5% mileage error when compared to a GPS system.
#8
06-25-2007, 09:42 AM
Re: accuracy of GPS mph compared to speedometer
The 3 mpg high reading on the speedometer matches my experience also. It is my understanding that this is pretty common. Motorcycles typically have an even higher overshoot, often as high as 10%
#9
06-25-2007, 04:04 PM
Re: accuracy of GPS mph compared to speedometer
I am resurrecting a post from another thread regarding the diagnostics screen's speed indicator (and its variance from the speedo reading):
Well, I found the article from C&D that I referred to (five years ago - I have a mind like a steel trap 
-- or a steel sieve if you ask my wife):
Linky: http://www.caranddriver.com/features...r-scandal.html
Actually my theory that this is the wheel sensor is that it is labeled "SPD". I thought that this was a sensor in the wheel that could be used for "dead reckoning" when the GPS signal is lost. This way it estimates your location on the map by using your speed/distance traveled, coupled with the gyro sensor that gives a relative bearing.
That brings up another question. That same diagnostic screen shows the relative bearing from the gyro sensor, but this never seems to match your compass heading. I am curious what this value is actually relative to. Is it relative to the direction you were going when you entered diagnostic mode, or is it relative to the direction you were going when you started out in (when you started the car)? I think it is the former, since the latter would be tricky if you started out in reverse...
I think that the speedo probably uses the input from this SPD sensor, but for some reason it is calibrated strangely to cause the low readings.
I recall reading somewhere about speedo error, and that European cars had their speedo calibrated in such a way that they NEVER read low, even if someone puts a huge set of wheels/tires on the vehicle. This is due to some European regulations that stipulate this "NEVER low" requirement. Perhaps Japan has a similar mandate? (I think this was from a Car and Driver editorial column a few years ago).
That brings up another question. That same diagnostic screen shows the relative bearing from the gyro sensor, but this never seems to match your compass heading. I am curious what this value is actually relative to. Is it relative to the direction you were going when you entered diagnostic mode, or is it relative to the direction you were going when you started out in (when you started the car)? I think it is the former, since the latter would be tricky if you started out in reverse...
I think that the speedo probably uses the input from this SPD sensor, but for some reason it is calibrated strangely to cause the low readings.
I recall reading somewhere about speedo error, and that European cars had their speedo calibrated in such a way that they NEVER read low, even if someone puts a huge set of wheels/tires on the vehicle. This is due to some European regulations that stipulate this "NEVER low" requirement. Perhaps Japan has a similar mandate? (I think this was from a Car and Driver editorial column a few years ago).
-- or a steel sieve if you ask my wife):Linky: http://www.caranddriver.com/features...r-scandal.html
#10
06-25-2007, 04:55 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.
