Can someone explain to me why my vehicle speed according to my speedometer and my speed according to my GPS/SatNav are out of sync by about 3 km/h? And which is more accurate? Thanks in advance.
In: Physics
A speedometer is connected directly to your car and is more accurate. With sattelite it’s basically guessing where you’re at by radio and stuff
In Australia it’s illegal for speedos to read low, but they’re allowed to read high by up to 10%+4km/h. Other countries are similar. So all the cars I’ve driven in far enough to test read high by about 5%. I test by timing the 5km speedo test markers on some roads which I assume are accurate and the GPS readings seem to match these results.
In other words, GPS is more accurate than a speedo for legal reasons. Interestingly, while speedos read high, odometers are much more accurate but read very slightly low. I’m guessing there’s more law behind that.
It could be either one.
Speedometers work by counting tire rotations, which can be affected by wear, inflation and temperature. Also, some manufacturers adjust their speedometers to read high so you can’t blame them if you are caught speeding.
A typical GPS reads your position once per second, if you are on a curve, it can cut the corner and make it seem like you are going faster. But they also can be off by several meters, giving a zig-zag path when you are really going straight, and reading slower.
A well-calibrated speedometer will beat most GPSs, but a GPS will beat most speedometers.
Most cars read low on the speedo. They do it on purpose so if you speed by 5 km/hr you’re not actually speeding by 5. GPS is much more accurate. The speed you see on a GPS is what you’re doing and therefore what a police officer would clock you at using a radar gun.
Edit: I meant read high. If you’re indicating 60
You’re likely only doing 57-58
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