Sat Nav speed reliability

[Davva]

Does anyone know how reliable/accurate the speed readout is on a Sat Nav?

(I have a Sony version about 6yrs old - if it makes a difference)

 

Well, when my Sat Nav states I am travelling 70mph the speedo is on 78mph.

I understand there is an accepted variance of approx 10% on a normal speedometer, so my 70 could really be 63-77.....? Is there a variance on Sat Nav speedo readings? From the road signs I did actually seem to be travelling slower than 70mph when the needle was on 70mph....... Sorry, boring I know but I travelled 600miles over Easter week/weekend.

 

Which should I believe? :unsure:

[Schern]

Satnav should be fairly accurate but will be affected by hills to a certain extent so it should be most accurate when your travelling on a flat road.

[lineshooter]

Ours reads 3mph slower in both cars, which I suppose is good if you`re approaching a speed camera.

[39TDS]

I reckon mine to be spot on, the vehicle speedo to be out.

Having had it in my lorry with calibrated tacho, it seems that would confirm it.

[bedwards1966]

I am of the view they are spot on, except perhaps when on hills which may affect them.

 

It's far from uncommon for speedo's to be that inaccurate, and I think that the 10% figure for variation is only at 30mph, I don't think there are any requirements for the accuracy at other speeds.

Most (if not all) vehicles read that they are travelling faster than they actually are, better that way I suppose than than the other way.

[apache]

They can never under read (ie say you are going 70 when you are doing 75)

 

Wikipedia is my source but I have read similar before

 

European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:

 

The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.

The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

[bouncer]

mine says i am doing 40 when my speedo says 44.

[southeastpete]

mine said i was only doing 120 when my speedo said 135...

[BigG]

I think the car manufacturers will always have the speedo overstate speed to ensure customers don't speed without knowing. If it were the other way they would be inundated by complaints.

 

My road angel says 70 speedo will say about 75.

[StuC]

Apache is correct. There is 10% over allowance but 0% under at ANY speed. Manufacturers always play it safe and leave a margin for error

[leeds chimp]

same as mind speedo pays 76 and sally says 70

[88b]

Apache is correct. There is 10% over allowance but 0% under at ANY speed. Manufacturers always play it safe and leave a margin for error

 

Correct, plus you need to allow for tyre wear. if you look at the speedo in an older car, they always has little dots before and after 30 and 70 mph for calibration. Police traffic cars used to have an additional calibrated speedo.

[activeviii]

change you tyres, change your speed. new tyres will read a different speed to wore tyres.

 

my old car was 8MPH slower that to different GPS units. my truck reads bang on, not that its a worry as it costs to much to speed :blink:

[88b]

change you tyres, change your speed. new tyres will read a different speed to wore tyres.

 

my old car was 8MPH slower that to different GPS units. my truck reads bang on, not that its a worry as it costs to much to speed :blink:

and that's why truck speed limiters are set up on barely legal tyres

[DSPUK]

My TomTom and Snooper S2 both read approx 4-5 mph less.

 

Dave

[Dunkield]

This comes up when people query the length of distance they have travelled clocked on a GPS, whether it's dedicated unit or built into a phone. The usual answer is consumer GPS/Sat nav isn't that accurate.

I use a GPS to track any walks or rides I do over about a mile or so, some days the output is bang on the money, other days it puts me in a middle of field or down the middle of a river.

A few people accused a local half marathon course of being over length according to many GPS readings at the end of the run, but they were all rubbished as the course is actually measured to the nearest yard/metre/meter/whatever.

[topshunt]

A few years ago the speedo in an artic i was driving started playing up so i used the speedo on the satnav whilst driving through 30mph zone with lots of speed camera's. never got any speeding fines, whether they worked or not i couldn't tell you. But as i believe sat nav works off three satelites so there accuracy in speed should be spot on.

 

Also in my car at 70mph satnav reads 65mph

[39TDS]

I am happy to believe my sat nav through any speed camera.

[Dunkield]

I think they are good enough, but certainly not as accurate as some are claiming.

Found the link with the really intersting looking people :blink:

 

http://coursemeasurement.org.uk/gps.htm

[evo]

:lol: :lol: :lol:

mine said i was only doing 120 when my speedo said 135...

[UKPoacher]

Simple way to test either is to maintain 60mph either by sat-nav or speedo on a motorway and time the mile posts passing by. At 60mph it should take 60 seconds to travel one mile.

 

In my Subaru Forester the speedo was always 10% more than the sat-nav. Whereas the same sat-nav is roughly the same reading as my Skoda Octavia.

[Imperfection]

In my van i have a speed camera detector,TomTom and the vehicles built in twatnav (its **** hence why i use TomTom) and they ALL read the same speed which is approximately 5mph less than speedo.