whats more accurate? sat nav or car speedo?

[simonruss]

hello, what is more accurate at telling your speed, a sat nav or the cars own speedo? as on a recent journey there was nearly a 10mph difference between the two. thanks

[terence7556]

if the sat nav was fitted afterwards surely that is more accurate, because these shops know where the measured miles are in the local area and take the vehicle
to set the sat nav against the set mile

[markja]

bet your car was reading higher than the sat nav?
my garmin250 says i'm doing 50 when my car says 55
if you travel on the motorway or some dual carriageways there are small round markers at the side of the road. they start yellow and every 1/4 mile add a red quarter to them. so in 3/4 mile there are3 /4 red & 1/4 yellow. the police use these to check their speedo and mile-ometers i believe. one mile at 60 mph takes exactly one minute. get a passenger to time you.

[4candles]

Satnav.

Speedos aren't allowed to read slow, so the makers err on the side of caution.

My Fiesta used to read 7 mph faster than my GPS unit, and my current Focus reads 2� mph faster.

These figures agreed with every roadside speed display I've encountered entering a town or village.

[Clanad]

Depending on the brand and model of the Satnav you'll find the "refresh rate" varies greatly. For example, have you noticed in the nav mode, when passing adisplayed landmark, such as going under an overpass, that the car's position rarely mimics that desplayed on the monitor. Usually the car reaches the underpass before it show such on the monitor.
The refresh rate error is most noticeable when accelerating or slowing. In constant speed driving in non-hilly areas, the GPS will be the most accurate.

[chas2008]

i use my TomTom in 3 different vehicles, it shows about 5% difference in all of them..(slower) although being GPS based and should be more accurate i stiil feel more comfortable with the speedo reading...better safe than sorry..

[fo3nix]

The speedo will err on the side of caution (tell you you're doing 30 when you aren't quite, and not the other way round), but there's no way the satnav will be much more accurate.

For satnav to work well, it has to be a pretty clear signal (weather patterns can affect results). As mentioned above, refresh rates also affect the accuracy. And finally, the satnav system uses GPS, which for civilians is only accurate to a few metres (and can be changed easily by the US military if they so wish); it should be pretty accurate, but I would trust the speedo (assuming a fairly new car without issues).

[Disco Fever]

Ive had 3 different sat navs and they have all been about 10% out as it's a long way from your car to the satellites circuling above your head and back to your car again....

[RevFunk]

Satnav every time

Speedos can be +/- 10%

[Thunderchild]

revfunk speedos can be +10% only so you may be doing 27 mph but get told its thirty it won't tell you you are doing 33 when you are really doing 30

I just got a satnav and went to the doctors and back with my dad and accuracy was very poor it has a slow refresh rate an in village driving is awful at telling my speed sometimes almost told me i was on a different road, if a sat nav has a declared accuracy of 15 meters (as if the cheapos like mine were so accurate) how can they give a correct speed reading ? if 1st reading is 15 metres behind and the second reading 15 metres ahead thats a 30 metre error and at slowish speeds thats a lot

[EmEd1984]

it also varies from the wear on tyres, tyre pressure, the wheel rim size etc (as the tyre diameter reduces, there is less distance covered per revolution of the tyre), which is why it can be recommended to get it calibrated regularly.

http://en.wikipedia.org/wiki/Speedometer