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Accuracy of MPH Feature


Eswau
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Santa brought me a Garmin V for Christmas and family obligations and now work, have keep me from exploring a lot of the features of the unit. I did turn it on and take it with me on a 2 hr drive yesterday (more family obligations). I was surprised to see that the unit tracked my driving MPH. I noticed that it was consistently 3 mph different (slower) than my Honda's dashboard reading. My question is, which one is correct? My guess is that it is the GPS, but thought I would check with other users also.

 

Thanks,

Eswau 560 (OA-WWW)

 

[This message was edited by Eswau on December 26, 2002 at 06:48 AM.]

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We talked about this one before, but I'm too lazy to Markwell you. icon_biggrin.gif We have tested our van with our GPS and with one of those police radar trailers, and against a friends car. All showed our Speedometer was 3 MPH off. So we're assuming that the GPS is very accurate...........Now get out there and find some caches! And have fun!

 

worried.gif Children are natural mimics who act like their parents despite every effort to teach them good manners.

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It only means the speedometer itself is accurate. If you put larger diameter tires on the vehicle the speedometer will read slower than the vehicle is actually going. If you put smaller diameter tires on the vehicle the speedo will read faster than the vehicle is actually going. icon_eek.gif

 

The speedometer has no way of knowing what what tires are on a vehicle (Electronic or not.) other than what was factory equipped. icon_frown.gif

 

My company is a Delphi Authorized repair facility for GM instrument clusters, power train modules and radios.

 

I'm not Lost, my GPS says I'm right here....no over here......no over here.

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Calibration: Comparison to a known standard or accepted value.

 

The dirt-cheap way of doing this: using the fundamental measurements of distance and time (d/t); 1 stopwatch, some measured miles, and a little time.

Be consistent when starting and stopping the watch, and use the cruise control.

I've found my car has great linearity but a -1 MPH bias (offset) at all speeds--sane and otherwise. icon_biggrin.gif I also double check this bias against roadside radar trailers whenever I get the chance.

When I set my speed to an indicated 61 MPH, I'm confident I'm going very close to an actual 60. And that brings me to the GPS speed indicated by both my Merigreen and ST Pro. Both are within 1 MPH and usually indicate +/- a few tenths.

There is some variance, of course, but if my speedo were broke I would trust the GPSr to tell me true 95% of the time.

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we had a speed measuring Dyno which was powered by the vehicle tires and that was all it did. The majority of "new" cars we tested were reading 5 mph fast. when the speedo showed 65 you were actually going 60 and your odometer was clicking off a mile at .95 actual miles.

 

I'm not Lost, my GPS says I'm right here....no over here......no over here.

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It seemed like I was flying by everyone on the road - even when I am going "only" 72 in a 65 (I know - it's above the speed limit....). When I had to rent a car it subjectively seemed like I was going slower at the same speedometer reading.

 

When Santa brought me my GPS it showed that at speedometer reading of 72 I was actually going 77 mph. I've been REAL lucky..... What an eye-opener.

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i road tested my '92 ford tempo and found the

MPH of the gpsr to be within 1 MPH of the speedometer. haven't tested the odometer yet.

 

i then roadtested my '01 ford expedition and found the speedometer was 4MPH over the gpsr MPH indicator. i didn't believe it so i did a time measurement of 1 mile and kept the expedition at 75MPH according to the gpsr, and holy sheist, it was 48.33sec. that means the gpsr is right on and my expeidition speedometer is 4MPH over the real speed, the tire diameter has not change from factory original specs.

 

the odometer on the expedition clicks 1 mile at 1.03 miles. thats' great, that means i'm logging fewer miles than i have actually driven

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We just returned from a road trip in our '97 F-150, and the GPSr was dead on! We tried it against our speedometer, and also against the mile markers (you know, 60 seconds per mile is sixty miles per hour). We use an old Garmin 12, and are very happy with it's accuracy.

 

Dave and Tracie

 

Me ambivalent? Well..... yes and no.

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your going to see improvements in speedometer calibrations coming from the major manufacturers. The tolerances we have to follow in repairing instrument clusters is fairly precise. We are allowed +/- 3 mph variance at 60 mph accending and decending the scale. (thats 1-1/2 mph each side of 60).

 

No manufacturer wants a bad rap for getting speeding tickets.

 

I'm not Lost, my GPS says I'm right here....no over here......no over here.

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the speedos were certified for the car and equipment package as is. we use the same equipment on all autos. i understand your point, but the cars are certified to a standard for the equipment used.. not factory. you would easily argue out of a speeding summons based on pacing if we did not certify properly. good info applied to the wrong situation.

 

quote:
It only means the speedometer itself is accurate.

 

wrong - it means the speedo itself in conjunction with the current tires, and vehicle configuration is accurate. we don't rely on factory certified speedos.

 

quote:
If you put larger diameter tires on the vehicle the speedometer will read slower than the vehicle is actually going. If you put smaller diameter tires on the vehicle the speedo will read faster than the vehicle is actually going.

 

that part is correct... we don't make it a practice of changing equipment after recv a certification. lol. sometime peoples want to prove something wrong, in a trivial way makes me chuckle, but i digress.....

 

imho

 

wings_flag.gif

required reading

My first bible

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It has been my observation, while using my Garmin eMap and V, that speed accuracy is affected by altitude changes. For example, while climbing or decending, your linear speed may be 60MPH but your level speed, across the planet is reduced or something to that effect. For example, a rocket going straight up at 1,000MPH has an actual ground speed of zero. But on a level surface, I would expect any GPSR to be as close to 100% accurate as anything would be. Three MPH off would surprise me and make me question how accurate the source is. But that is only a hobbiest's opinion.

 

Steve Bukosky N9BGH

Waukesha Wisconsin

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The speedo on our GMC Envoy reads right on the money with my GPSV and the radar trailers we have past. Today we tried it with our new GPSmap 76S and again the speedo and the 76 were right on the money. So if A=B and B=C then im goign to assume the V and the 76S were both getting it right. Next time ill have both the V and the 76S in the car at the same time and see what happens.

 

66427_2800.gif

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My Magellan GPS says 63 and the Ford Bronco 2 says 65. I believe the GPS. I agree with the post 2 before myself that says the auto makers don't want a bad rap for causing a speeding ticket.. My sister has an Ford Exporer and her car was same according to her Magellan GPS.. her Ford was about 2 MPH slow in the speedometer.

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quote:
Originally posted by Steve Bukosky:

speed accuracy is affected by altitude changes. ...while climbing or decending, your linear speed may be 60MPH but your level speed, across the planet is reduced or something to that effect.


Steve, while this is technically true, the difference between your ground speed and speed indicated by the GPS is surprisingly small.

 

Assume a 7% grade hill (which is very steep for any paved road). Climbing the hill at 60mph, your true horizontal speed (the speed which your GPS would indicate) is 59.85mph.

 

Even with a 15% grade hill, which is very rare in my experience, driving up the hill at 60mph, your GPS would indicate 59.32mph

 

Even hiking in mountainous terrain, the differences are very minor.

 

Real-world elevation changes have very little effect on horizontal speed or distance.

 

Jamie

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A couple of things

 

First off. The one point missing here is that calibration happens at a particular speed. I.E. A speedo might be calibrated at 55. At 25 it might be off, at 85 it might be off, but at 55 it is calibrated.

 

Also, if the car is German engineered, you will likely find the speedo is 5 km/h off (give or take) This is alledgedly due to stiff manufacturer's fines (in Germany) if the speedos read low.

 

YMMV (and speed too icon_biggrin.gif)

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Previosu cars of mine have been so close to the GPS (within a needle's width or so) that it was insignificant. My new Outback is off from the GPS by 2.5 miles per hour, which according to my dealer is within the +/-4% they allow for. My odometer is also off by 2.5%, that is, one part in forty (odo says I've gone 41 miles when I really only went forty)...this bothers me more than the speedo because it means my mileage warrenty will end prematurely.

 

BTW, I've check the speedo/odo to the best of my ability against the mile marker posts along side the Turnpike. The GPS and markers match exactly, the car is off as described.

 

ApK

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I just ran a highway test with my '02 GMC Envoy and my V and my wife's new 76S. It was spooky. The Envoy's speedo and the V and the 76S acted as if they were all wired together. All 3 were spot on within +/- 1 second of their screen updates. So much so that the V and the 76S were also updating their map pages and all their other indicators within a second of each other.

So I am going to assume that my Speedo and the GPS's are all reporting the correct MPH. Now there were small differences in Lat/Lon, very small differences. Ill assume this due to the fact i was using the external roof top antenna for the V and the 76S's antenna buitin on the dash. The V had a higher signal strength, but when out in the clear both GPS's showed identical values for their estimated position error.

 

66427_2800.gif

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When we got our GPS in March 2002, we were driving a rental car while our car was in the shop. I saw the cool speedo and decided to test it out on the rental car. The rental car spedo read the same as the GPS Speedo. I would do 70, and the GPS said 70. When we picked up our car and were driving home. We decided to do the same test. The car speedo said 70, but the gps said 68mph. I kept doing tests and my car's speedo was always off by around 2mph. After that I did not think too much of it until the day I got pulled over.

 

It must have been 3 or 4 months ago, and I was driving from Ellensburg WA to Yakima WA. Of course I knew what my GPS said. My car's speedo said 80. When I got into Selah, which is right before Yakima, the State Patrol had a speed trap set up. They pulled me over and gave a warning. They told me that I was going 78 which is exactly what the GPS said countless numbers of times before. If I would have been going 80, they would have given me a ticket. From that point on, I do not doubt my GPS. In fact, I have even had a friend ask me to test their car's Speedo.

 

Yep, they are very, very accurate. Which brings me to another story. There was a gentleman who rented a car, and was charged a huge bill by the rental agency for speeding. See, the rental car had a built in GPS unit that they used to keep track of the position and speed of the car and if it went over x number MPH then you'd get billed for it. I guess the caught him on the highway speeding. Anyway, when he came to return the car they slapped him with the bill. if I'm not mistaken, the guy fought it, and won and they had to refund the speeding penalty money.

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quote:
Originally posted by RossOlson:

What about when you drive slower or faster. Wouldn't a tire become taller and skinnier the faster it spins? Could this change the accuracy of speedo at different speeds.


 

I've wondered also about tire pressure.

I suspect that difference by these things would be too slight for the average commuter with a Timex to measure accurately.

 

ApK

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I echo Centuar on the Garmin MAP76S. Mine seems to be within 0.1MPH of my vehicle's analog speedometer (2002 Ford F150 pickup). It seems to show direction change very quickly also. I can't complain! Today I had a "personal best" accuracy of 8.9'. All this happened with the unit being held in my hand in the vehicle's cabin (I don't recommend doing this!).

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quote:

Also, if the car is German engineered, you will likely find the speedo is 5 km/h off (give or take) This is alledgedly due to stiff manufacturer's fines (in Germany) if the speedos read low.


 

My friend's BMW reads 5 MPH under the GPS speed, while my Chevy just about matches the GPS.

 

homer.gif

"Just because I don't care doesn't mean I don't understand."

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Most of the previous comments on accuracy are relative...

 

To get it right you need constants..

 

Mileposts and time are constants.

 

The most accurate test of any car and/or gps unit is going to be by finding a level stretch of road, and locate county or state mile posts along side of road....

 

If you are going 60mph by car speedo, and going 60mph by GPS, you will be doing the mile in 60 seconds...

Got stopwatch.....

 

Dale

 

--------------------------------------------------------

I'm Diagonally Parked, In A Parallel Universe.

--------------------------------------------------------

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quote:
Originally posted by BassoonPilot:

quote:
Originally posted by sbukosky:

... For example, a rocket going straight up at 1,000MPH has an actual ground speed of zero.


 

This is only true on "The Day the Earth Stood Still."


 

Ok...what about 'an ant climbing straight up a wall...at 1,000 MPH.....'

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A number of points:

 

Here in the UK, the law specifies the margin of accuracy for speedometers. It's something like 10% at 30 mph and 10% at 60 mph etc. However, the speedometer is not allowed to read slow i.e. if the speedometer says 60 than that is the maximum speed the car can be going at. For this reason, speedometer manufacturers deliberately make them fast (or so I've heard). If you have a speedometer that could be anywhere between +5mph and -5mph of the true speed, you'd have to throw away half of the speedometers you make. If it's +/- 5mph of 5mph less than the true speed, you can keep them all. Therefore, in a British car expect your speedo to be wrong.

 

Second, most modern tyres have a steel belt running around them under the tread. This means, the tyre will not get bigger as it spins faster unless you count thermal expansion of the belt as the tyre gets hotter. For the same reason, the circumference should not get smaller if the tyre is not fully inflated (the steel belt is still the same length, just a bit floppier).

 

Third, as I understand it, GPS receivers measure speed by analysing the doppler shift of the satellite signals in the same way as police radar guns. This means that they measure your true speed. If you are travelling straight up at 1,000 mph, the GPS receiver will say "1,000mph" not zero mph. Note that car speedometers do the same thing. If it was possible to drive straight up a wall at 1,000mph, the speedometer would say "990mph" or something like that.

 

-------

jeremyp

The second ten million caches were the worst too.

http://www.jeremyp.net/geocaching

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According to the technicians at Garmin, they use a combination of measuring the doppler shift and checking the time to travel from one position to another, when they calculate the speed. Probably since they normally display a two-dimensional speed, i.e. exclude the vertical component of the speed.

 

By the way, when driving up that wall at 1000 mph, you speedo would show something like 1050 mph, right?

 

Within the European Union, since January 1st 1996, cars have to have something know as "Complete Car Authorization" (don't know exactly what it's called in English). If not, the car can't be sold within the EU. But with such an authorization, it can be marketed in all countries within the EU, without any special national authorizations.

These rules state that the speedo should be accurate -0% +20%. That allows for all different tire dimensions and so on that could be used on the car.

 

Anders

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quote:
Therefore, in a British car _expect_ your speedo to be wrong.


 

Of course, why would the speedo be different than any other part. Of course, the speedo in a British car is probably very accurate most of the time, reading zero, while the car sits in the mechanic's garage.

 

Kidding. Just kidding. icon_wink.gif

 

ApK

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