GPS Speedometer which one is correct ?

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b_beder
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GPS Speedometer which one is correct ?

Postby b_beder » Sat May 09, 2009 2:56 pm

While driving my path Speedometer display 100 Km but GPS Speedometer display 90 there is 10 km difference. is that normal
Do i have to calibrate Speedometer?


Gonzo
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Postby Gonzo » Sat May 09, 2009 3:53 pm

I'm not 100% sure on this but my instincts would tell me to go with the GPS. It uses multiple satallites to triangulate your location, speed, etc. Your speedometer is mechanical and has a some variables that can throw it off by a small percentage. The faster you go, the higher the percentage of your speed to your speedometer that will be off. Does that make sense to you? Just my .02 cents.

Gonzo

Mark72501
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Postby Mark72501 » Sat May 09, 2009 6:27 pm

In my previous vehicle, it always read 2-3 mph slower than the GPS would show.....and I think the GPS was right. Just my input...not scientific or any data to back it up other than my personal experience. Haven't put the GPS in my new Path yet so I don't know if there's a discrepancy in it yet...

cbsmith
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Postby cbsmith » Sun May 10, 2009 3:55 am

My 2008 SE also reads almost exactly 10km/h slower on the speedo than on GPS. The GPS speed matches up to what a radar indicates so I think the GPS is correct and the speedo is off.

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Captain
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Postby Captain » Sun May 10, 2009 8:13 am

Yes this is normal. if you do a search I wrote a topic on this.

LittleStevie
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Postby LittleStevie » Sun May 10, 2009 10:13 am

Go with what the GPS says. The vehicle's speedometer relies on assumptions of tire diameter in calculating a linear distance from number of tire revolutions. Treadwear and tire pressure could contribute to error.

I've observed the opposite of Mark, on every vehicle I've seen the speedometer reads 2-3 mph faster than the GPS. I would not be surprised if the automakers do this intentionally for conservatism, to avoid getting sued by someone who says their speedometer caused them to get a ticket.

Mark72501
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Postby Mark72501 » Sun May 10, 2009 12:19 pm

LittleStevie wrote:Go with what the GPS says. The vehicle's speedometer relies on assumptions of tire diameter in calculating a linear distance from number of tire revolutions. Treadwear and tire pressure could contribute to error.

I've observed the opposite of Mark, on every vehicle I've seen the speedometer reads 2-3 mph faster than the GPS. I would not be surprised if the automakers do this intentionally for conservatism, to avoid getting sued by someone who says their speedometer caused them to get a ticket.
My apologies, I meant to say 2-3 slower on the GPS...got distracted and mis-typed...so, same experience. And your reasoning makes sense. Same reason most law enforcement officers won't stop a vehicle for going 5mph or less over the limit...room for error/calibration of the radar equipment & the speedometer. Makes sense...and actually is a little comforting to know the vehicle is reading 2-3 faster than actually moving... :)

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NVSteve
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Postby NVSteve » Mon May 11, 2009 9:06 am

One of the biggest culprits is that the mfr calibrates the speedo for one size tire. On the Pathfinders with 16" wheels, that means everything is calibrated to 265/75/16 tires (which is the tire size indicated in the OP). But, the second new tires start rolling, the diameter begins to shrink as the tread gets worn, which also contributes to the problem. As others suggested, I'd stick to what the GPS says.

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GOTGOO
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Postby GOTGOO » Mon May 11, 2009 3:21 pm

My speedo reads 2-3mph faster than my gps at 80 mph, but im also running the 245/75/16's tires still... Dont know if that helps..

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08Datsun
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Postby 08Datsun » Mon May 11, 2009 5:27 pm

GOTGOO wrote:My speedo reads 2-3mph faster than my gps at 80 mph, but im also running the 245/75/16's tires still... Dont know if that helps..
That's how mine was with the stock tires. Now that I've switched to 265/75/16, my speedo is absolutely spot on with the GPS.


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