Sat nav calculates your moving speed from plot point to plot point. It's only absolutely correct if you're a) moving in a straight line, b) moving on the level, c) moving at a steady speed, d) it can 'see' the satellites. Contributor who said check it on a rolling road... well, it wouldn't matter how fast your wheels were turning, your vehicle would be travelling 0mph and that's what your satnav would show!
My satnav consistently indicates my X-Trail's speedo is over-reading by around 10%, but for the above reasons, I wouldn't use it as a speedometer- reading is too small for glance-use anyway and therefore unsafe. My previous car's speedo was apparently much nearer the mark- an MG ZS180, and I definitely noticed other cars going 'too slow' past speed cameras. I was using the same satnav device. Better safe than sorry when it comes too points on your licence and cash out of your pocket.
At a constant speed of 60mph my road angel reads 56, i dropped to 50mph for 3 miles and my road angel reads 46. I increased to 70mph for 7 miles and my road angel reads 66. I increased to 75mph for 2 miles and my road angel reads 71.
Speedo seems to be reading less than my road angel by 4mph.
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Mick + Tina
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Your car speedo will always be inaccurate, as even if it had been calibrated at some point, differences in tyre pressures, and tread depth adjust the circumfrance of the tyre, and there for the actual speed of the car.
Its for this reason the police traffic cars are supposed to be callibrated frequently, and why most small cop cars cant stop you for speeding as they aint callibrated.
Interestingly the speedometer on my car (bmw 330i) exactly matches the sat nav so for example if the speedo is reading 50mph so is the satnav. I've tested this from 20 mph to over 70 and they are consistently the same.
In my wifes car the sat nav read 3-4mph slower than the speedo.
When I said you can take satnav with you to a rolling road, I meant get the car calibrated first then set the satnav up so that you can see which is correct.
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Quote: Originally posted by Stevieg330 on 03/10/2007
Hi,
Interestingly the speedometer on my car (bmw 330i) exactly matches the sat nav so for example if the speedo is reading 50mph so is the satnav. I've tested this from 20 mph to over 70 and they are consistently the same.
In my wifes car the sat nav read 3-4mph slower than the speedo.
Cheers
Steve
Likewise, my BMW 330d reads exaclty the same as my TOMTOM 520
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Topic: sat/nav/speed
