GPS Speedometer Accuracy

[Guest bunkerdave]

I am just wondering if anyone has any thoughts/insight/concrete knowledge on the accuracy of GPSRs as speedometers. Mine always reads a little lower speed than my car speedometer (so naturally I go by the GPSR ) and I am wondering if there is any reason not to.

 

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BunkerDave

N 40° 7.081, W 111° 38.851

[Guest Lazyboy]

I think they are very accurate, on flat ground. Seems to me they'd be a bit slow on any hill.

 

Or am I wrong again?

[Guest mrgigabyte]

You should read this recent thread on this exact subject.

 

http://forums.Groundspeak.com/ubb/Forum13/HTML/001076.html

[Guest madphatboy2]

BD,

Drive around until you find one of those tattletale cops (the ones that tell you what your speed is digitally) and see if your GPS matches it.

 

 

[This message has been edited by madphatboy2 (edited 12 February 2002).]

[Guest Groves_Trekkers]

Or get on the freeway and go 60 miles per hour and time how long it takes you to go between two mile markers. If it takes longer than 60 seconds then your speedometer is off scale high. If it takes shorter than 60 seconds, then your speedometer is off scale low.

[Guest rdwatson78]

quote:

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Originally posted by madphatboy2:

BD,

Drive around until you find one of those tattletale cops (the ones that tell you what your speed is digitally) and see if your GPS matches it.

 

 

[This message has been edited by madphatboy2 (edited 12 February 2002).]

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Find a cop with a radar gun and ask him to get your speed as you drive by. I'm sure that would get an interesting reaction.

 

rdw

[Guest YaesuMan]

Mapleton City just got a new little toy, they stick it around town, come on over and check.

(but personaly I would trust a GPS more)

[Guest Gee Ode]

I've heard that your car speedometer can change if you have changed the size of your tires. I have used my GPS unit while driving and it is right on. My van has the original size tires. Pat C.

 

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Pat C. nmc491@aol.com

[Guest SNIFTER]

My car is out 10%. I took it back to the yard and asked them to check it but to no avail. So if my speedo reads 100 I am only doing 92. We have checked this against 2 GPSs. The car yard wanted to know how we thought that it was out. We told them and they agreed that the GPS would be the more accurate instrument. When I get my next radar ticket I will let you know.I hope to keep you all waiting a very long time.

[Guest Gliderguy]

My Garmin III+ is right on with my wife's Saturn as far as speed goes. Even up and down hills of maybe 5% grade.

 

[This message has been edited by Gliderguy (edited 16 February 2002).]

[Guest Trafcon]

xact all the time and the older ones are the worst. Better to look for one of the speedometer check locations on the freeway.

 

I also took RDWatsons advice and had a motor cop check mine with his LIDAR radar gun. I made five runs and was right on each time. Within 1 mph at 60. ( It helps that the cop is a co-worker of mine.) []

 

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KTF !!! GBWY !!!

[Guest Trafcon]

I have checked my Garmin V using the milepost method and it was fairly accurate. Be aware that the mile markers are not exact all the time and the older ones are the worst. Better to look for one of the speedometer check locations on the freeway.

 

I also took RDWatsons advice and had a motor cop check mine with his LIDAR radar gun. I made five runs and was right on each time. Within 1 mph at 60. ( It helps that the cop is a co-worker of mine.) []

 

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KTF !!! GBWY !!!

[Guest CYBret]

I had been concerned about my eTrex Venture not being right . . it always seems to be about 3-4 mph behind the speedometer in my car. However, today I used it in my wife's car and it was dead on. So, I guess I'm gonna pick up the speed when I drive now!

 

Bret

 

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I wept because I had no hat, then I met a man who had no head.

[Guest BassoonPilot]

quote:

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Originally posted by Gee Ode:

I've heard that your car speedometer can change if you have changed the size of your tires. I have used my GPS unit while driving and it is right on. My van has the original size tires. Pat C.

 

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It's true . . . my wife and I have identical cars, but I always wondered why my car seemed to cruise a little (2-3 mph) faster at the same RPM. When she had a flat one day, and I went to put the spare from my car on hers, I found out the reason . . . her car has 13" wheels and mine 14". I guess it's a minor enough discrepency that the manufacturer doesn't bother to use a different speedometer gear. (Incidentally, the gps says my car is moving a little faster than the speedo ... the Mrs. car is spot-on.)

[Guest bunkerdave]

I did the odometer thing on my trip to SE Utah on Thurs., and at 100 miles per the car, I had exactly 97 on the GPS. So I guess I can figure the GPS /Car error ar 3%. Not that it is really important in the car, since the only time I really use the GPS odometer is on hikes, when it is the only odometer I have. I still have the 2-3 mph variance between the car gps speedometer, however.

[Guest JAMCC47]

Dave:

I seem to remember that you wished to get in shape for backpacking this summer. So try this one. You hold the GPS, your wife drives the car, the kids can giggle while you run along side the open window, and you two can compare readings.

 

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JoseCanUSea

[Guest pewterz]

It is quite common for the speedometer in a car to be off by a few MPH. As for changing the size of your tire's effecting your speedometer It depends on the overall diameter of your wheel not your rim size. For example you can have a 13 inch rim with a normal tire or a 14 inch rim with a low profile tire and still have the same total diameter.

[Guest apersson850]

Here in Sweden, legislation used to say that speedometers should be within +/- 10%.

Now they've changed that to -0% / +20%, just to make sure that it never shows too little.

 

Anders

[Guest King Pellinore]

well my GPS and my car agree speedwize, and that's good enough for me. Overall distance can vary if the GPS loses connection briefly. Speed when I'm walking however is sometimes right, but if the reception gets funny it's all over the place (I may be old and slow, but I can definitly run more than 1.1 mph!!)

[Guest JamieZ]

quote:

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Originally posted by apersson850:

Here in Sweden, legislation used to say that speedometers should be within +/- 10%.

Now they've changed that to -0% / +20%, just to make sure that it never shows too little.

 

Anders

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Goofy Swedish laws. Wasn't it you guys that switched sides of the road not too long ago? In the late 60's or something.

 

Jamie

[Guest Geoffrey]

My GPS units: GPS 3,3+, and 5

These units usually say about 3 miles an hour over what my car's speedo says.

 

Most consumer GPS units dont calculate the altitude gains or losses in the speed calculation.(At least the older ones.)

 

Has anybody compared driving on completely flat land versus mountain driving???

 

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Geoff's GPS Information Page

 

Geoff's Information on GPS units, and price guide for several units

[Guest bunkerdave]

ort. Every other weekend since, I have been in SE UTah hiking in Grand Gulch. 8 miles one saturday, 15 the next,(friday and saturday) and 10 this weekend. This has been hiking around looking at ruins and rock art, no caching, but that will come this summer. I was pretty tired/sore after this last hike, so I guess I am not in the shape I want quite yet. One thing I have learned is what I can do without in my pack, and I have it down to 38 lbs. with water now. Amazing what you can learn to do without, when it is strapped to your back.

 

bd

[Guest bunkerdave]

quote:

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Originally posted by JAMCC47:

Dave:

I seem to remember that you wished to get in shape for backpacking this summer. So try this one. You hold the GPS, your wife drives the car, the kids can giggle while you run along side the open window, and you two can compare readings.

 

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Heh -

 

Well, I have been making an effort. Every other weekend since, I have been in SE UTah hiking in Grand Gulch. 8 miles one saturday, 15 the next,(friday and saturday) and 10 this weekend. This has been hiking around looking at ruins and rock art, no caching, but that will come this summer. I was pretty tired/sore after this last hike, so I guess I am not in the shape I want quite yet. One thing I have learned is what I can do without in my pack, and I have it down to 38 lbs. with water now. Amazing what you can learn to do without, when it is strapped to your back.

 

bd

[Guest apersson850]

quote:

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Originally posted by JamieZ:

Goofy Swedish laws. Wasn't it you guys that switched sides of the road not too long ago? In the late 60's or something.

 

Jamie

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Yes, we came in together with the minority, that drives to the right, on September 3rd, 1967.

Besides, since the all-European approval scheme for cars was introduced, the same goes for the entire European union. (Now I'm talking about speedometers, not which side of the road you drive on.)

 

Anders