Accuracy of Garmin GPS in measuring speed

[MFB]

Hi guys,

Given that there are factors that contribute to inaccurate readings (see http://www8.garmin.com/aboutGPS/), is this more accurate compared to standard units of measure for speed? Automotive speedometers are believed to be inaccurate compared to gps readings on a clear day and out in the highway. Please share your views here.

Info below is from the garmin link above:
Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

* Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
* Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
* Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
* Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
* Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
* Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
* Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

[Hardright]

Car manufacturer's overstate the speedo to ensure you stay below the speed limit with allowance. Otherwise, they could get sued for all the speeding tickets levied on car owners. The variance between my Garmin and the speedo is about 10-12 kph. So in my opinion, gps reading over a two minute stretch would be an accurate reading given all the factors you indicated. 2 mins at over 200kph is an eternity.

[MFB]

So does it mean going over 200 kph and reading it less than 2minutes via gps will not be accurate? Whats with the 2 minute stretch? Thanks

[oj88]

2 minutes might be an exaggeration, CMIIW. A few seconds of running at a constant speed is usually enough to stabilize the GPS reading. Once stabilized, the speed reading is pretty accurate.

Most commercial GPSes aren't fast enough in updating speed readout when there's a lot of acceleration and deceleration involved as they're not built to calculate that fast (to save battery). I imagine that precision or mil spec GPSes are the most accurate ones.

[Tantantiniiin]

pretty accurate for me.

Try doing a comparo with an all-stock car, single passenger, no load, and correct tire pressure. You will note that the reading is pretty accurate with the same reading. Try hovering around 70-80(on your speedo) at NLEX, coastal then compare with your GPS screen.

Once I placed it on my other car with new tires (a little oversized from stock 14 to 17). The speedo would register something like 78-80, but the GPS would rate it to around 82-85kph. I went to http://www.carbibles.com/tire_bible_pg2.html to compare the original speed and the new speed with the new tires, and it comes out squat with 85kph.

[DaveMorrison]

Bet your licence on it

Speed from GPS is very accurate (less than 1km/hr) error from my experience, provided you have 5-6 satelites. The innacuracies in the postion is largely due to atmospheric conditions but these tend to drift fairly slowly. To get the high accuracy commercial positions via GDPS or RTK means you have a base station monitoring the drift and updating (via radio generally) the remote units every few seconds. So relative position epoch to epoch is very good.
Normally I drive a company car with a vehicle monitor in it. These are normally set up via GPS or hard measured distance and should never be done via a speedo. Speedo's are always 5-10% slower than true speed if using manufacturers wheels and tyres. My speed via GPS is always very consistent and when limited by the monitor to the national speed limit, I never get speeding tickets, even in places such as Egypt where there are no error margins given. I have driven in a convoy where the others cars have picked up numerous speeding tickets and I never got one.

You should try logging a air journey, the accuracy of the speed is remarkable. Honest - just put your Garmin at the window of a commercial plane and you get good lock!

[_LO_]

pretty accurate for me.

Try doing a comparo with an all-stock car, single passenger, no load, and correct tire pressure. You will note that the reading is pretty accurate with the same reading. Try hovering around 70-80(on your speedo) at NLEX, coastal then compare with your GPS screen.

Once I placed it on my other car with new tires (a little oversized from stock 14 to 17). The speedo would register something like 78-80, but the GPS would rate it to around 82-85kph. I went to http://www.carbibles.com/tire_bible_pg2.html to compare the original speed and the new speed with the new tires, and it comes out squat with 85kph.

Hello sir, I was able to test the speed accuracy as well. The same set-up you have, from 14" OEM wheels to 17". the difference was only around 2 to 3kph from the car's speedometer. In one of the events that I attended, we drove up to 210kph and the reading in the GPS is still within the same variance as that of the lower speed

[woohoo]

The Speedometer on my Nuvi 255W is very consistent with my all-stock MPV.

[Hardright]

Reason I say 2 mins is that it gives the gps enough time to determine the ground speed despite all the factors that might affect it (ionosphere, clouds, buildings, number of satellites, etc). Also, it doesn't matter what tire specs or mods were made to the car. The objective was to measure ground speed. Also, wasn't looking at measuring speedo and gps variances at 75-85kph. Was looking at speeds of 150-200+ kph variances. I see many YouTube posts showing speedos past 200kph. But did they really achieve 200? A gps reading in my opinion is better as it negates the mods done to the car.

btw these are not safe speeds in our roads. Best done on a track or drag strip.

[nandyg1]

my garmin would only show 267 kph when my speedo indicated 281kph......

[Hardright]

The 267 is the accurate number.

[oj88]

my garmin would only show 267 kph when my speedo indicated 281kph......

281 (267) Kph? What do you got there, a Porsche?

BTT: Anyway, car speedometers are mostly calibrated to be 5 to 10% faster than actual speed. This has caused somewhat of an outrage from some car owners claiming that manufacturers are conspiring to end the warranty on their cars sooner than expected. So if the warranty runs out when the odo reads 100,000 Km, your actual traveled distance is probably just in the 95,000 Km. This doesn't cause much of a fuss here since seldom do our cars reach 100,000 Km in 3 years.

[d1754]

I observe during our trip to Bagiuo that the reading on our Estrada is 140kph when we are driving in NLEX and SCTEX but my gps reading is only 110kph.

[tomgotan]

The gps is more accurate than the speedometer in dash. i used Obd2 computer interface to scan the speed and it is the same as the gps reading. the only different is the speedometer in the dash is 2 to 3km higher.