Speedometer Error
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New User
Joined: Dec 2003
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From: Deep South
Speedometer Error
On the way home from my first weekend trip in my '04Screw, I noticed the speedometer did not agree with my GPS, which was sitting ontop of the dash. The speedometer was reading 2-3 mph faster than the GPS.
The truck was taching about 2,000 rpm .. speedometer read 72-73 .. and the Garmin III+ GPS read 2-3 mph less than the truck, tracking 8 out of 12 satellites, and varied between 68 and 69 mph.
My Lariet does have 18 inch tires. I don't have a clue if that caused the difference.
Anyone have any insight?
The truck was taching about 2,000 rpm .. speedometer read 72-73 .. and the Garmin III+ GPS read 2-3 mph less than the truck, tracking 8 out of 12 satellites, and varied between 68 and 69 mph.
My Lariet does have 18 inch tires. I don't have a clue if that caused the difference.
Anyone have any insight?
Elder User
Joined: Jan 2004
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From: N Ga. Mountains
I'd say that was pretty close considering one is a direct measurement of your wheel rotation and the other is taken from satelites 22,500 mi above the Earth. If they were both exactly the same I would be shocked.--Jack
Senior User
Joined: May 2003
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From: Baton Rouge, LA
My GPS reads about 2-3 mph below the speed on my speedometer. If you read up on GPS systems, it'll tell you something like how the federal government requires some degree of error in the GPS satelites for national security. I read that sometime before 9/11. Im sure they still include the error today. This is a site I just found...scroll down to where it says "Selective Availability" for more information. http://www.romdas.com/technical/gps/gps-acc.htm
Junior User
Joined: Jan 2004
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From: Cypress, TX
Concur with Bassaway. Army veteran here - there is a mandatory 10 meter minimum error thrown into all commmercial GPS receivers. It also affects velocity readings, but I do not remember the precise specs on that error. If you get your hands on a military one however... Well, the black helicoptors will start circling overhead since those same satelites that emit the readings for the GPS to receive, can also triangulate on a military GPS for the purposes of finding POWs or stolen military equipment.
Senior User
Joined: Oct 2003
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From: Midland, TX
I was about to post something very similar to this. The other night I started my truck while holding the trip reset button because I wanted to use the digital speed readout. It turns out that the digital readout is 2-3 mph slower than what the analog gauge shows. I'm guessing that the digital readout is more accurate than the guage. I thought this was pretty weird...
Now I see why I haven't gotten a ticket yet! (I usually travel 5-7 mph over).
Now I see why I haven't gotten a ticket yet! (I usually travel 5-7 mph over).
Elder User
Joined: Jan 2003
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not to be cynical, but if speedos/odometers were routinely set by (any) manufacturer to read slightly fast, the consumer would perceive quicker acceleration times, vehicle would report better miles per gallon of gas and the manufacturer would effectively reduce the actual warranted miles from the stated 36,000.
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From: iowa
My Garmin GPSIII+ reads within 1MPH of my speedometeron my '04 screw. On my GMC van (company vehicle) the speedometer is about 3 mph slower. Yes there is some error built in, however once the sats are locked in, I believe the error is constant therefore the speed should be acurate
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New User
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Umm...they analog guages are not set to read fast or slow, high or low. However, realize that the pointers are actually installed by hand onto the instrument cluster and there is a tolerance on this operation.
Signed,
Your friendly neighborhood engineer who actually worked on this cluster at Visteon
Signed,
Your friendly neighborhood engineer who actually worked on this cluster at Visteon
Senior User
Joined: Jan 2004
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I think a Highway Patrol or Trooper's radar would be very accurate.
Kevin, it's nice to see Ford still does a few things by hand. It's interesting that there is a tolerance with the pointer. Too bad Ford won't give us a tolerance on our warranties when we are a few miles over. I once had a major 3/36 warranty item denied at 36,045 miles on another Ford. 2-3 mph difference can add up to a lot of miles.
BTW, isn't the odometer digital? And what's up with the cluttered numbering on the speedo?
Kevin, it's nice to see Ford still does a few things by hand. It's interesting that there is a tolerance with the pointer. Too bad Ford won't give us a tolerance on our warranties when we are a few miles over. I once had a major 3/36 warranty item denied at 36,045 miles on another Ford. 2-3 mph difference can add up to a lot of miles.
BTW, isn't the odometer digital? And what's up with the cluttered numbering on the speedo?
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Selective Availability (which degraded GPS accuracy) was turned off back in May of 2000. A quality GPS today can be accurate to within a few feet.
Any east way to check is to follow AG4.0's advise and use a stopwatch.
As far as the conspiracy theorists are concerned, I have to believe that if a manufacturer can build a speedometer thats within 1/2% of the real speed, and the law requires them to be within say 5%, they would set them up to run 4% or so fast. This would give them a 1/2% cushion and still let them realize all the benefits that 03f150man pointed out.
Any east way to check is to follow AG4.0's advise and use a stopwatch.
As far as the conspiracy theorists are concerned, I have to believe that if a manufacturer can build a speedometer thats within 1/2% of the real speed, and the law requires them to be within say 5%, they would set them up to run 4% or so fast. This would give them a 1/2% cushion and still let them realize all the benefits that 03f150man pointed out.
Elder User
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ultramax, I have no idea if the Feds have a standard for odometer accuracy but with all the cost/profit squeezes put on the manufacturers it is a reasonable question to ask how accurately the odemeters read. new subject---KevinMac , what is the point (no pun intended) of your post? it doesnt matter where the pointer is, or even if the gauge has a pointer. we are talking recorded milage. Why do you mention analog--new vehicles are (generally) digital. So ultramax, back to your point. To have a lawsuit, it would be interesting to know if there was a federal accuracy standard for the odometer, whehter the manufactuer violated the standard, and/or whether the manufacturer systematically and intentionally rigged the odometer reading so that the manufacturer was within the letter of the law while also purposely screwing buyers out of their warranty. It would be easy to collect a statistically valid sample of a given vehicle make/model/type to determine if the cars were consistently configered to run on the high side. If there was a high correlation, then that would invite more questions.
Last edited by 03f150man; Mar 1, 2004 at 11:28 PM.
Elder User
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there are several posts here from Very New Users. Any one else out there i.e. any established users who care to comment ? Racer guy, Flash,etc etc. i.e. people who have previously often posted here that we also know. Big Red Power, you are not specific. What are you saying about your odometer accuracy? are you saying it is normal to read high or low or accurate? And i dont understand jberkshire commetn " yes there is a built in error" ??? "once satellite is locked in, the error is constant and the speed should be accurate" how can an incorrect speed become accurate? i will agree it is possible to be consistently inaccurate but that doesnt make it correct.
Last edited by 03f150man; Mar 2, 2004 at 12:51 AM.