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Which is more accurate - the car speedometer or satnav? The car one consistently says I am travelling 3mph faster than the satnav thinks I am. Top of the range satnav. Just not sure which one to go by.
 

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Satnav.

Speedo depends on tyre pressures. I do have my tyres hard so bigger diameter, maybe yours are a lower pressure? Also tyre wear needs to be borne in mind. Check your tyre pressures.


If the wheel diameter is small then it must turn more times in a given distance than a larger one. The speedo can only measure distance traveled by wheel turns taking into consideration the gear used. It is also possible Ford have calculated the speed based on the gear used and the wheel diameter being a middle one, so the speedo will vary ever so slightly.


I always go by the slowest (except road works), which is usually the speedo in my case.


I forget now what the allowance for discrepancy is when thinking in terms of speed limits.


In restricted road works areas where there are cameras I turn on the cruise control and use the buttons to get match my speed to the satnav. I usually overtake other cars very slowly. That suggests that other cars may be using their speedo and being a tad under is safer than over.

Edited by: makem
 

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I agree that Sat Nav is the best indicator of speed, I use both a Garmin and Tom Tom simultaneously, and they always agree with each other, 30 m.p.h. on them equates to 32 m.p.h. on the speedo. I never even thought about the effects of tyre wear and pressure on the speedo readings, but I suppose it is a logical consideration. With the Constabulary adopting zero tolerance in order to keep the speed awareness frauds active, we need to be very aware of our speed.
 

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If for example, I set my speed to 50mph with cruise control and wait until it has settled, then check satnav, I always find that I can add 2 or more mph to the speedo to match the satnav. Means my speedo lags, seems yours is ahead.
 

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I have no expertise in these things but have noticed the same discrepancy whilst driving various hire cars. I had always presumed it reflected the fact that sat navs calculate on the journey made along a rise of straight lines (track of maps and also periodic fixes of position), whilst the speedo calculates based on the real road journey of all the curves.
 

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Yes, for journey planning and time to destination the satnav calculates from map data in the unit.

However, the realtime measure of speed is taken from the distance between two points (or more) over time taken to travel that distance. The distance between the two points is down to the resolution of the satnav satellite connection. In surveillance it would be millimeters and for us I think in terms of meters.


I know what you mean about straight line navigation and find it a pain to be taken off a major road at a flyover/bridge and then back on.
 
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