gps speed - how accurate

[Stevie]

Anyone know how accurate the speed is with gps?



Have TT2 on Ipaq 3970 using Globalsat BT308 and the indicated speed shows about 4mph slower than the card speedo? Read somewhere that car speedos are usually around 8% out does that mean the gps is more/less accurate?

[trafcam]

If you're driving at a steady cruising speed and your GPS has a good signal, then the GPS speed should be much more accurate than your car speedo. If you're under hard acceleration the GPS speed will lag behind you.



Car speedos generally run high by design, reading as much as 5% over your actual speed. Also keep in mind a car speedo is based on the circumference of your wheel and tyre combination, which can alter by a significant distance as your tyres become more worn and the circumference decreases.

[Lost Cause]

I take it that TTN2 assumes that you've travelled in a straight line, between any two given points, to calculate the speed ? Hence the GPS speed would appear to be a little slow, if you were travelling around a long bend, for instance ?



In this particular situation, the car speedo might be more accurate ?

[__dg]

Where does the accuracy (or inaccuracy) of the GPS speed come from?

Is it from the satelite, the reciever or software? Or a bit of each?



I use TomTom and an Ipaq in two vehicles - one speedo shows a consistant 8mph lower, the other 5mph lower than the GPS reading.



Now, if I was to get stopped which reading should I rely on and would the Police accept my GPS reading instead of my speedo reading?



That is of course, assuming they catch me first!

:o))



dg

[Darren]

Are yon sure your car speedometer reads slower than your GPS? Because they cannot by law read under, most manufacturers set them to read approx 5% over your actual speed. As long as your speed is constant (regardless of whether you are driving in a straight line or around a curve) then the GPS speed readout will be as near to exact as possible.



Innacuracy creeps into the calculation if you are accelerating/decellerating because the GPS will only update once or twice a second (this is a software issue) and hence it's readout will lag behind your true speed.



If you were stopped for speeding by the Police they would care not whether you were driving to GPS or Speedometer speed. They wouldn't be interested in you unless you were travelling at considerably more than 5% over the posted speed limit and Vascar, Laser, Police Pilot, calibrated speedo etc are accurate enough that even allowing for operator errors you'd be booked.




_________________
Darren Griffin

[Darren]

Are yon sure your car speedometer reads slower than your GPS? Because they cannot by law read under, most manufacturers set them to read approx 5% over your actual speed. As long as your speed is constant (regardless of whether you are driving in a straight line or around a curve) then the GPS speed readout will be as near to exact as possible.



Innacuracy creeps into the calculation if you are accelerating/decelerating because the GPS will only update once or twice a second (this is a software issue) and hence it's readout will lag behind your true speed.



If you were stopped for speeding by the Police they would care not whether you were driving to GPS or Speedometer speed. They wouldn't be interested in you unless you were travelling at considerably more than 5% over the posted speed limit and Vascar, Laser, Police Pilot, calibrated speedo etc are accurate enough that even allowing for operator errors you'd be booked.




_________________
Darren Griffin

[Stevie]

My speedometer is reading over the gps speed. The brief tests I did were on a straight 2 mile road at a steady speed.



It was curiosity rather than anything else, I've known about the inaccuracy of speedometers but never had anything to compare them with. Never been done for speeding and never needed to argue the case.

[DavidW]

I just wanted to link to an older thread on this topic (which, despite being in the TomTom Navigator 2 forum, is about GPS speeds) - take a look here.



I hope this is helpful.







David

[alix776]

ok here goes trucker view i tend to use cruise control most of the day so my speed is fairly constant thank those wonderful muppets in brussels only resticting most trucks to 53/54 mph i know some go faster but im just having rant down to business

police calibrated speedos have 3% plus minus i know this because they are calibrated useing the same calcutation that vosa uses to caslibrate tachographswhich is why most traffic cars are fitted with vascar

my speed is usually set around 1-2 mph out on my speedo this depends on how new the tyres are my car speedo is 5 mph slow which is almost constant through out the speed

range

also a word of advice if stopped by a police officer and there is no vascar only a speedo ask for calibration certificate if this can not be provided it is more likely that the speeding ticket will be thrown out the are usually right but can get it wrong sometimes hope this helps


_________________
currently using aponia truck navigation on windows phone. Good bye IOS don't let the door hit you on the way out .

Oh the joys of being a courier.
device Lumia 950 xl

[__dg]

Quote: Originally posted by __dg on 14 August 2003

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I use TomTom and an Ipaq in two vehicles - one speedo shows a consistant 8mph lower, the other 5mph lower than the GPS reading.

---

Ooops, did I say lower? I meant higher





In terms of traffic law, I assume that I am required to have some sort of speed recording device - ie the fitted speedo. But could I potentially remove this and rely on my GPS to tell me my speed?

Would this be lawful? Would a court accept that my GPS reading was just as accurate as a callibrated Police device?



Cheers



dg

[alix776]

no sorry under constution and use regs you must a type approved speedo
_________________
currently using aponia truck navigation on windows phone. Good bye IOS don't let the door hit you on the way out .

Oh the joys of being a courier.
device Lumia 950 xl

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