how reliable is GPS speed?

[seanj]

i have tomtom nav2 which tells me i'm doing around 10mph less than my speedo?

can gps be this far out, or should be be getting my speedo calibrated??

[Stevie]

Seanj it really all depends on the strength of signal you are getting, the more sats that you have the more accurate your postion and speed will be.

Sean,
I have noticed this on mine aswell..with a good satellite fix, it seems to be about 10mph slower than what my car says im doing!!
i'd like to know whether my car or GPS is telling me the right speed !?? Anyone have any ideas?!

[seanj]

well the obvious solution (which i'm yet to try) is to get some one to drive at 30 in front of you... then when you've checked you speedo,flash them to go 40.. check your speed then flash them again to try 60.. etc...

with all this wonderful technology, something just need a bit of good old brain noodle and common sense...!

the main reason i rasied this point is because i drive an imported japanese sports car that was converted from kph to mph and i was a little concerned...

[blokeupnorth]

Peugeots are having serious problems with speedo's in particular the 307, not sure if it was the actual speed but certainly the mileage. It was reported on watch dog!

http://www.bbc.co.uk/watchdog/thisweek/index3.shtml could this be the problem?

[nickbooth]

From what I have found out on this subject is that most cars speedo's can be up to 10% out. Also if you think about the accuracy percentages, if you are running a gps accurate to say 30ft then in a hundred yards time it is still likely to be accurate to 30ft but the gps should have still logged a movement of a hundred yards ish

[iaveight]

Originally Posted by seanj

i have tomtom nav2 which tells me i'm doing around 10mph less than my speedo?

can gps be this far out, or should be be getting my speedo calibrated??

the amount of Sats you pick up will have a little bearing on how accurate it is but remember this, you couldnt take one of those units into and airplane and expect it too tell you your speed...Reason is the components in it. certain GPS systems are made for hikers, put it in a car and they wont read anywhere near accurate, put one for a car in an airplane and it wont read accurate. It has to do with the components in the unit it self. Check out the unit at low speeds and it will read accurate now speed up and you will find it will be out by approx 5 mph or so, its not your speedo its the hardware in the unit.....The max sats that a gps reciever will use is 4 this is called a 3d fix the cars use 3 sats which is called a 2 d fix.....

[StevieMcM]

GPS Speed is far more accurate than the speedo in any car.

Rolling resistance and tyre expansion (When the tyres are warm) will affect speed, nd can be as much as 10%.

That's when you see any max speed test's of any vehicles, always look for the GPS Verified speed.

The only down side of the GPS speed is that it's not as quickly updated as the speedo.

[iaveight]

thats my point, its the gps software that is to blame for the faulty reading in speed not the car speedo. An Aircraft GPS is accurate at 450 kts to about 5 knots and that takes into account wind, pressure differences, temp changes, and much more. You cant expect the same accuracy from a car GPS for a few hundred quid then to an aircraft GPS worth a few hundred thousand.....The TOM TOM I used the last few days was a great tool for finding your way around and wrong turns ect. Speed is good for telling the other half what time to put dinner on give or take 5 minutes....Other half being male or female of course

[roycollingwood]

I use tomtom3 in a hyundai, the gps speed and speedo speed are 'more-or-less' the same, within 1 or 2 mph