accuracy of GPS miles per hour compared to speedometer - GreenHybrid

acco20 · #1 (**permalink**) 06-24-2007, 03:10 PM

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 miles per hour reading accurate.

McGyver · #2 (**permalink**) 06-24-2007, 04:07 PM

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 miles per hour 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?

David Price · #3 (**permalink**) 06-24-2007, 04:19 PM

acco20: I checked my wife's TCH against a Magellan 315 GPS last fall and found that the GPS read 67 miles per hour and the speedometer on the car read 70 miles per hour.
Not sure how accurate the GPS is.

McGyver · #4 (**permalink**) 06-24-2007, 04:28 PM

Quote:

Originally Posted by David Price

acco20: I checked my wife's TCH against a Magellan 315 GPS last fall and found that the GPS read 67 miles per hour and the speedometer on the car read 70 miles per hour.
Not sure how accurate the GPS is.

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.

Sallyspal · #5 (**permalink**) 06-24-2007, 04:49 PM

Quote:

Originally Posted by acco20

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 miles per hour reading accurate.

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 miles per hour "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 miles per hour's of leeway when I become concerned about the speed limit on a particular road.

David Price · #6 (**permalink**) 06-24-2007, 06:22 PM

By the way; the odometer read 61 miles and the GPS read 61.9 miles.

flopshot · #7 (**permalink**) 06-24-2007, 07:36 PM

Check out this post from last year:
http://www.greenhybrid.com/discuss/s...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.

chris_h · #8 (**permalink**) 06-25-2007, 09:42 AM

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%

spiff72 · #9 (**permalink**) 06-25-2007, 04:04 PM

I am resurrecting a post from another thread regarding the diagnostics screen's speed indicator (and its variance from the speedo reading):

Quote:

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).

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

Pete4 · #10 (**permalink**) 06-25-2007, 04:55 PM

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.