ChatterBank3 mins ago
whats more accurate? sat nav or car speedo?
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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
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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.
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.
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.
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.
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).
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).
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
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
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
http://en.wikipedia.org/wiki/Speedometer
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