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rjscooke3

GPS: Garmin Nuvi 760 & Nuvi 260wt







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This is all subjective i believe for several reasons on unless you have your car speed tested at a recognized testing facility you would not know if it reads right or not. Secondly I doesn't ever remember reading any data from Garmin as to the accuracy of there units, nor do i ever expect too.

Regards Russell
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johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3







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rjscooke3 wrote:
This is all subjective i believe for several reasons on unless you have your car speed tested at a recognized testing facility you would not know if it reads right or not. Secondly I doesn't ever remember reading any data from Garmin as to the accuracy of there units, nor do i ever expect too.

Regards Russell


Good for you Russel... Think I would rather trust a GPS's speed reading over a suspect speedo reading, if there was a discrepancy between the two, as it does not seem to matter what brand GPS you take a reading from, they all seem to come up with the same results.
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bjujitsu

GPS: Nuvi 1490T







Post GPS Speed disparity     
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Speedos have to read slightly faster than the vehicles actual speed to eliminate the chance of speeding imagine the outcry if people where fined for speeding due to the manufactured speedo reading being slow.
Also motor manufactures like the fact that they have to make the speedo read faster because it makes the fuel consumption look better, service intervals are shorter, it gives the impression that everything lasts longer in terms of mileage, and lots of people change their cars when it has done a certain amount of miles...
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johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3







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bjujitsu wrote:
Speedos have to read slightly faster than the vehicles actual speed to eliminate the chance of speeding imagine the outcry if people where fined for speeding due to the manufactured speedo reading being slow.
Also motor manufactures like the fact that they have to make the speedo read faster because it makes the fuel consumption look better, service intervals are shorter, it gives the impression that everything lasts longer in terms of mileage, and lots of people change their cars when it has done a certain amount of miles...


Not sure which country you are in, but it is not law that speedos read faster than true in Australia, as the national standard seems to be 10% allowance and the onus is on the driver to make sure you don't break the law. Unfortunately different states has differing views as to the speed error they allow before booking someone. In Victoria it seems to be down to 5% error and open your wallet.
Most GPS units have an accuracy readout that will depend on how many satellites it can see and how good the unit is.
The 2 Garmin's I have go down to 3 metres accuracy some time and stable around 9 to 12 meters, so I would trust the accuracy of the GPS every time.
People tend to forget the speedo reading will change if you alter tyre size or have very worn tyres, change rim sizes etc etc etc.
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rosspat

GPS: KNA-G430







Post GPS Speed Disparity     
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I drive a Toyota Land Cruiser which had Cooper now Goodrich tyres which are not standard size. This affects my speedo reading by approx. 4% slower when compared with the GPS. In fact the Vic Roads overhead speed checkers and the GPS generally are the same. I generally drive to the GPS speed indication. So if your tyres are standard then the issue is most likely the speedo variation.
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sebastian

GPS: iPhone 4s







Post Re: Car speedo might be off     
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schmidklaus wrote:
I have the same speed discrepancy between my in-car speedometer and the GPS. My GPS reads a bit higher.

Here is the explanation: The car's speedometer is geared from the car's road wheels and most people don't realize what this means. The speedometer installed in your car is designed for the specific tires that the vehicle is originally equipped with. The circumference of the tires affect the speedometer calibration. If you replace/change the tires for a different brand and/or size - I wanted a wider tire for better wet and winter traction - the ratio of road distance travelled to wheel rotation may change slightly. When I wanted to put a better tire on my mini-van, I sat down with my tire dealer and the tire manufacturer's catalogue/specifications and compared the tire diameters. Of the tire series I wanted to put on, there were none whose diameter EXACTLY matched the diameter of the original equipment tires. I replaced P215/65R17 tires with P225/60R17 and ended up with a tire diameter 0.4 inches (1 cm) smaller. This results in a speedometer reading 1.5% lower.

The GPS, since its not based on mechanical wheel rotation, may be more accurate than the speedometer - I certainly treat it as such.


All correct!
Without considering the performance loss due to the more friction on the road...Wink
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tboneandporcini

GPS: Samsung Galaxy Note 2, rooted, PACMAN ROM, Copilot Live Premium







Post GPS accuracy is pretty high     
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I guess, the main question here is how accurate GPS is. It is pretty accurate - various sources say it is about +-0.1 to +-0.5 mph. However, the trouble is that GPS readings are displayed with some lag, which may affect the readings when accelerating/braking/turning.

Speedometers in the cars are also accurate, but often tuned to display a higher speed. This is to trick the drivers and slow them down. In sports cars speedometers are usually spot-on, unless wrong tyres are fitted.
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glenwen

GPS: garmin nuvi 2797







Post Speedo Inaccuracy     
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Our cash strapped governments are always willing to find ways to seperate you from your money.

My daughter purchased a 20 year old car, in excellent condition.
This car was manufactured when speedos had a 10% allowable tolerance either way.
She was booked for doing 7 klm over the speed limit when she was doing the speed limit by the speedo.
I checked the speedo and it was reading 9klm under the actual speed.

I approached the powers that steal, and they stated most clearly that it was up to her to ensure that the vehicle speedo was correct.
In other words, again, the driver is responsible for all things that they can get cash from you.

She payed, I gave her my old tom tom and set it up so she could monitor her speed.

Its a shame the governments waste so much time enforcing minor speed issues, perhaps if they concentrated on actually fixing the goat tracks they call roads, they would save some lives.
But that's just pie in the sky thinking, cash is king and speed is an easy way of raking in the cash.
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johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3







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Very unusual for a speedo to be that far UNDER actual speed. Most errors read the other way round (show 100 and actual 95 etc) Are the tyres still the original size as when car was new ?
Different size tyres and / or rims will give incorrect readings..
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Guest








Post GPS speed error     
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Tested on multiple Toyotas in South Africa and the speedometers are always around 10km faster than the GPS reading at 120kmh.(tested with multiple Garmin and Tom Tom Gps units)
Possibly the motor vehicle manufactures are scoring on warranty ( 10% on 100 000 km actually means a warranty of around 90 000km)
Just a thought though







njsmith622

GPS: Garmin Nuvi







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I have a nuvi 2595 and I trust it more than my speedometer. When I am driving 80kph my speedometer is only off from my gps by 3-4kph but when I am going 120 it is off by 6kph. So either I have the wrong tire size on the car or they build cars with a some leeway built in.
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john.blandford4

GPS: Garmin nuvi 3597







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steven.chaffer3 wrote:
My Garmin Nuvi 1390t consistenlty shows my speed to be 5% lower than the speed the car's speedo is showing.

At 100 km/hr on the car's speedo the 1390t is showing 95 km/hr. Several times when this is happening I have checked the satelite screen to see the accuracy which has always been at 3 metres. By my reckoning, a 3 metre error in a km (1000 m) is 0.3% which should amount to a 'error' of 0.3 km/hr at 100 km/hr, effectively zero.

I can't believe my car's speedo is 5 km/hr per out. (However I will check it next chance I get.)

Any ideas about what is happening?

Cheers

Steven



Your GPS is the one you can trust. It is accurate.

Your car has many factors built in to help ensure that you don't exceed the speed limit. Almost all speedometers have a tolerance built in, generally forcing them to read a little 'high' so that if you are for instance sitting on '100' by your speedo, you should actually be travelling a little slower. This is mandated by safety regulations in various counties. In Australia it is an 'ADR' Australian Design Rule.

Also the laws of physics come in to play. Your speedo is (through either electronic or manual gears etc measuring revolutions of drive shafts and thereby wheels. The circumference of your wheels alters (minutely, but measurably) under different pressures and as your tyre tread wears out. This also is 'absorbed by the 'tolerance' built in above.

Your GPS is physically measuring the speed of your GPS unit from point to point continuously.

I drive to my GPS when cruising on the highway and confidently know the 'actual' speed I am travelling at. Have never yet got a speeding ticket using this method.
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b.shelton

GPS: Garmin Nuvi 3689







Post GPS versus car speedo reading     
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I have used three different GPS devices and they all show a speed 3 mph lower than the speedo in the car.
Car is a 2012 C250 Merc Coupe.
I would conclude that the speedo in the car is in error but I do not mind as it actually gets me to drive a little slower than I have to and therefore reduce likelihood of getting fined.
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BullwinkleII

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johmazz2006 wrote:
Thought the GPS used ALL available satelites, so shouldn't make much difference whether you go up or down hills, recon it would average out.


I think they use all the satellites to plot where you are, then check how long it took to move 1metre or whatever. But I doubt they take the elevation into account.

I hope they do.

Roads like the start of Adelaide's South Eastern Freeway would probably give enough error to get a speeding ticket if it isnt taken into account





jeffreies

GPS: Navi 1490T







Post Re: car speedo might be off     
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nimadude wrote:
You know your car speedo could be showing more than the real amount.. I know thats the case with my car - every time I pass one of those cameras that tell you the speed ur going (they dont take picture, just warn you), it shows my speed 5-10km/h lower than what my speedometer shows.. how old is ur car?


It may just pay you and place your GPS in another vehicle ..by rights your speed in the Subaru should be faster with your speedo then what your GPS is telling you..
Mine on 110Km GPS speed car speedo sits on 118Km and I go by my Garmin 24/7
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