Satnav speedo accuracy?

[paulco]

I recently purchased a Navman iCN 520 and found it has a speed facility. Does anyone know how accurate this is as the Navman shows 70 when my car speedo shows almost 80!! All the readings on the car are faster than the sat nav shows. Can I legally use the satnav as my speedo?

[Mortartube]

You will probably find that your satnav speedo is pretty accurate. Car speedos generally read higher than the real speed being travelled, so that if there is a slight innaccuracy, the manufacturer doesn't get sued for you getting caught speeding, when your speedo actually said that you were travelling at the speed limit. The police have accurate speedos that are frequently calibrated, whereas the normal car owner never calibrates their speedo from new to the scrap yard. Hence the over read

[BillyBB]

Doing time and distance tests my speedo was a full 10% over reading and the GPS was under by a 1%

[mallorn]

Speedo readings are also influenced by factors such as the inflation of tyres, which influences the circumference of the wheels.

And SatNav records lateral speed across the face of the earth as recorded by satellite, and makes no allowance for the extra distance travelled vertically (ie up and down gradients) which can mean you're travelling further and thus faster.

[nigel wilde]

hi i have a tom tom sat nav and that shows about 8mph slower than actual speed

[clancyblob]

My only experience of satnav/speed is on boats but the principles are the same. There is probably a lag between actual speed and satnav speed, as the satnav is talking to satellites every few seconds, comparing the change in distance from car to the satellites, and calculating what speed has resulted in these changes. If you are going round bends etc even at a steady speed, these calculations would be compromised. I would expect the most reliable comparison to be after a steady extended distance (several miles) in a straight line, on a moroway for instance. This lag effect would probably not be acceptable legally as a speedo, but that is only my "armchair" opinion!.