4 ms·

I'd expect it to be better than the car speedo. Car speedos typically overread by up to 10%, and if you fit a non-standard tyre, they can be way off.

Twirrim 8y ago
GPS is susceptible to atmospheric conditions, obstructed views (4 satellites minimum required for positioning) and a number of other factors, most of which are undetectable at the device level. Car speedos are done with over-read deliberately as a safety measure. Better overestimating than under. As a driver you can make conscious decisions about what speed you're driving based on all evidence available, including an understanding of how over/under your speedometer returns results. With a GPS based speedometer you can't realistically do that, you can have no idea what the accuracy percentage is at any particular moment in time. It might be over, it might be under. No one can make good decisions based on junk data.
GordonS 8y ago
I might have this wrong, but I seem to recall reading somewhere that speedos in the EU have to overread by some random amount, with the aim of curbing speeding.
gmueckl 8y ago
Soeedometers perform the most precise measurement you can get. When you fit a nonstandard tire you just introduce a constant scaling factor. That is not so bad. GPS on the other hand is atrocious. Don't even try to get an accurate position in places like downtown Seattle. I'm regularly getting misplaces by several blocks because of occlusion and reflections from the buildings. You just can't compute an accurate speed from such a signal. Besides, GPS can only update once a second. And given the signal jitter you can't get a good speed reading from the last two fixes - when you are going in a straight line. When you are not going straight, the GPS based estimate must underestimate the distance you travelled and thus your speed. Using this toy speedometer thingy for driving is just dangerous and insane and illegal in so many ways that I wish the article could just be pulled from the web.

Twirrim 8y ago

GPS is susceptible to atmospheric conditions, obstructed views (4 satellites minimum required for positioning) and a number of other factors, most of which are undetectable at the device level. Car speedos are done with over-read deliberately as a safety measure. Better overestimating than under. As a driver you can make conscious decisions about what speed you're driving based on all evidence available, including an understanding of how over/under your speedometer returns results. With a GPS based speedometer you can't realistically do that, you can have no idea what the accuracy percentage is at any particular moment in time. It might be over, it might be under. No one can make good decisions based on junk data.

GordonS 8y ago

I might have this wrong, but I seem to recall reading somewhere that speedos in the EU have to overread by some random amount, with the aim of curbing speeding.

gmueckl 8y ago

Soeedometers perform the most precise measurement you can get. When you fit a nonstandard tire you just introduce a constant scaling factor. That is not so bad. GPS on the other hand is atrocious. Don't even try to get an accurate position in places like downtown Seattle. I'm regularly getting misplaces by several blocks because of occlusion and reflections from the buildings. You just can't compute an accurate speed from such a signal. Besides, GPS can only update once a second. And given the signal jitter you can't get a good speed reading from the last two fixes - when you are going in a straight line. When you are not going straight, the GPS based estimate must underestimate the distance you travelled and thus your speed. Using this toy speedometer thingy for driving is just dangerous and insane and illegal in so many ways that I wish the article could just be pulled from the web.