How accurate is Sat Nav speed readings

wazza

I have noticed that the speed reading on the sat nav is lower than the reading on my car's speedometer. I know that it is acceptable for the car's speedometer to over read if that is the correct expression. If the speedo reads 30mph then it is okay for the car to be travelling at or below 30mph and not over.

How accurate is the reading on the sat nav. Can one use that at a guide?

Treadway1

I wouldnt suggest it as my TomTom tells me Im driving through central Dusseldorf at about 90mph at times. Also, if you get caught for speeding, then this would never stand up in court, as the argument would be that you had a perfectly good speedo in front of you to use.

With regard to why it shows slightly slower speeds, its probably because the technology just isnt there right now to accurately read precisely how fast your moving (Well, it may be, but certainly not for a couple of hundred quid GPS system).

HTH

*EDIT* Thought Id better point out that I dont live, nor have ever visited Dussledorf, which is why I find it strange that Tom lands me there from time to time.

hewhoisnotintheknow

Treadway1 wrote: »

I wouldnt suggest it as my TomTom tells me Im driving through central Dusseldorf at about 90mph at times. Also, if you get caught for speeding, then this would never stand up in court, as the argument would be that you had a perfectly good speedo in front of you to use.

With regard to why it shows slightly slower speeds, its probably because the technology just isnt there right now to accurately read precisely how fast your moving (Well, it may be, but certainly not for a couple of hundred quid GPS system).

HTH

*EDIT* Thought Id better point out that I dont live, nor have ever visited Dussledorf, which is why I find it strange that Tom lands me there from time to time.

i think thats down to poor signal, making it jump around all over the place, hence the high speed shown

ariba10

The average car speedometer reads about 10% more than what you are actually doing.

ie. 33 = 30 . 55 = 50.

Treadway1

hewhoisnotintheknow wrote: »

i think thats down to poor signal, making it jump around all over the place, hence the high speed shown

Oh I realise that. Thats why I dont think its a very reliable method for speed reading.

vikingaero

It's fairly accurate, more so if the road is level-ish (not up or downhill) and fairly straight.

Most muppets seem to travel past a 40 mph speed camera at an indicated 35mph. Which in reality means they are doing low 30's mph. Frustrating.

Treadway1

vikingaero wrote: »

It's fairly accurate, more so if the road is level-ish (not up or downhill) and fairly straight.

Most muppets seem to travel past a 40 mph speed camera at an indicated 35mph. Which in reality means they are doing low 30's mph. Frustrating.

More annoying are the 'accidents waiting to happen' who drive at the speed limit, only to jump on their brakes 2 metres before a speed camera, despite being legal anyway!!!! :mad:

Huskie_2

Sat-Nav speed readings are considerably more accurate than almost any dashboard speedo - even many digital ones.

Sat-Navs work by timing the diffence that it takes for one sattelite signal to reach it compared ro several others, the system is kept in time by a global clock that is broadcast by the sattelites. As the Sat-Nav system knows where every sattelite is it knows that if it receives signal 1 first, then 2 and 3 at the same time it's closer to 1 and equidistant from 2 and 3 - bit of simple computing and you have your location anywhere on the earths surface and with considerable accuracy. The speed calculations you get - with a decent quality Sat-Nav is never really more than a couple of miles different from a police laser gun, and usually within fractions of the speed.

The reason sat-nav systems (in general) show a slower speed is that they are far more accurate than your avarage speedo. The speedo works by measuring wheel revolutions and that would work fine if tyres never wore down or people never bought slightly bigger wheels or tyres - so the manufacturers build in a significant margin of error, which gets bigger as your speed increases.

A friend of mine is a copper and he's tried it in normal cars with the laser guns and an indicated 77 can be as low as 70 - within the legal limit. And the sat-Nav inside the car was showing bang on 70/71. On one occaision, and he tested TomTom and Garmin - the Sat-Nav out from the speed gun by more than a couple of miles - whilst one older car was a full 10 miles an hour slower than the speedo, he thought he was at 70, Sat-Nav said 61 and speed gun said 60.

Just a final thought - with more Sat-Navs around and more speed cameras it would be useful for people to know that if the keep to the limit on their speedo they'd have to be very unlucky to be knicked.

MH;)

Pssst

I suspect that the car speedo is more accurate.

After all,it is directly linked to the vehicle which is directly linked to the road via its tyres. In addition,the car speedo instantly records changes in speed wheras the satnav has a time lag.

Which is more likely to be accurate? One directly linked or one in which a radio transmitter and receiver are over 22,000 miles apart?

photome

Pssst wrote: »

I suspect that the car speedo is more accurate.

After all,it is directly linked to the vehicle which is directly linked to the road via its tyres. In addition,the car speedo instantly records changes in speed wheras the satnav has a time lag.

Which is more likely to be accurate? One directly linked or one in which a radio transmitter and receiver are over 22,000 miles apart?

Without question as the previous poster said the satnav is the most accurate.

Pssst

photome wrote: »

Without question as the previous poster said the satnav is the most accurate.

Why?.....................