How accurate are Sat Nav speed clocks

[gwh200]

It has to be to a certain extent in order for any kind of sensible accuracy for navigation.

The inaccuracy latency produces is in the noise when it comes to velocity calculation.

if you took this to its natural conclusion and did a bit of easy calculus
you would eventually reacha a rate where your speedo would read (approx) 66% faster than you were going!!!

unfortunately that speedis about 18000mph(ish)

so its only stein who can prove it!!!!!

[plumb bob]

All very nice, but the bl00dy thing still sends me in the wrong direction.

[JUDWAK]

used my tom tom in all kinds of motors its never gives the right reading on speed gets worse on motor ways faster ya go bigger the gap from speedo to tom tom its ok at 30 or so

[Gupster]

Like half of you have said i wouldnt go by what it says. Using my total standard astra van with standard size wheels \ tyres my sat nav (navman) speed is always out, the faster i go the more it is out by. Borrwed a mates tom tom before i got mine and that was out by more & the palmtop versions are even worse because of the processor \ memory power.

Like onlynik said there is a tx \ rx delay + processing time + sat nav accruacy.

Best way to check you speedo is by getting someone to drive next or behind you (as long as you know there speedo is good).

Cheers,

[Taliesins]

Like half of you have said i wouldnt go by what it says. Using my total standard astra van with standard size wheels \ tyres my sat nav (navman) speed is always out, the faster i go the more it is out by. Borrwed a mates tom tom before i got mine and that was out by more & the palmtop versions are even worse because of the processor \ memory power.

Like onlynik said there is a tx \ rx delay + processing time + sat nav accruacy.

Best way to check you speedo is by getting someone to drive next or behind you (as long as you know there speedo is good).

Cheers,

Not a very accurate way of getting a true reading as speedometers are allowed to be 10% out 10% over that is, as they are not allowed to read below., most speedometers read at 110% but they can vary anywhere within that 10%. One can assume therefore that no two (standard speedometers) would give the same reading at say 70mph as they could read anywhere from 70 -77 mph.

[onlynik]

Besides, a GPS unit doesn't tell you how fast you are going it tells you how fast you were going.

It knows where you are.
You move.
Unit talks to satellite, work out distance from old position to new position and the time you've taken to get to the new poisiton.

It can then work out the speed you've taken to move.

In the time to talk to the satellite you could have moved another 20 metres.

[bensonsnuts]

Yes , but it's been mentioned before if you use a gps in a straight line at a constant speed you will get a fair reading . Any time delay should not matter . Yes you may get a reading that is 100m behind your position , that is why you maintain the same speed

[plumb bob]

[QUOTE=Best way to check you speedo is by getting someone to drive next or behind you (as long as you know there speedo is good).

Cheers,[/QUOTE]


The best way to check your speed is to find a speed camera in a 30mph zone and drive through it repeatadly starting at 40mph and each run increase your speed by 10mph until you reach 70mph.

Then all you have to do is wait for the tickets to arrive with your actual speed printed on it............job done and accurate enough for the police to gain a prosicution by.

Alternatively find a motorway cop and drive past him at 100mph and when he catches you, he'll politely ask you if you know how fast you were going, then he'll tell you EXACTLY how fast you were going!


This can be a costly exercise, but as we all know, accuracy costs.