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Discussion Starter · #1 ·
Hi everyone, am new to the forum and i was wondering if anyone can give me some tips on what would cause my speedo to show 70mph when it's actually doing 62-63mph (friend alongside and sat nav reading)
Thanks
Andy
 

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Hi, most speedos are out by a small percentage and the figures you quote are not unusual.

Hopefully a brighter spark than me will be along shortly to explain why!!
 

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Hi Andye

I imagine there has to be some error on the speedo, if for no other reason than the wheel (tyre) diameter changes as the tyre is worn.

But how accurate is Sat Nav. What it does for route finding is impressive but does it measure the speed against the inbuilt mapping or from point to point . Driving technique can make a difference on a journey distance. For example, on the motorway I suspect you'd have to middle lane all the time for any mapping system to be reasonably accurate.

At least the speedo is only counting the revolutions of the wheels.

Xacre
 

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xacre said:
But how accurate is Sat Nav.
More accurate than your speedo. It will calculate the speed using GPS and providing you've got a decent signal should be very accurate.
 

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Hi RJW

I'd agree that it should calculate extremely accurately the speed you have travelled from A to B. But there's a lot of bends in the road.

When I measure a map for a walk say it might say 6 miles. It will probably take me 3 hours but I may well have covered 7 or 8 miles in that time. You need to measure every yard you cover to get an accurate reading of the speed you travel at.

So does a Sat Nav measure its points in 1 metre, 10 metre or possibly 100 metre bits for the purpose of speed readings

Xacre
 

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My last X-Trail (a 2.2 dci) speedo was overreading by 10%. I could'nt understand why everybody was going past me on the motorway when I was doing 70mph,and on my bumper when sticking to 30 going past the speed cameras !.I checked it against a hand held GPS wich also tallied with the average speed reading on the built in sat nav. I was told all car speedos are set between 2-10% overread to prevent the manufacterus from being sued . It would cause real problems if they underread.Also it allows for tyre pressure variation and temperature. Gps is accurate to 0.01 MPH with a four satellite "lock on "
My new one overreads by 2% so I am happy with that.
 

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xacre said:
Hi RJW

You need to measure every yard you cover to get an accurate reading of the speed you travel at.

So does a Sat Nav measure its points in 1 metre, 10 metre or possibly 100 metre bits for the purpose of speed readings

Xacre
What's a Yard, I remember my Grandad used to mention them !!! :lol:
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As said by "Jezzer747"
Gps is accurate to 0.01 MPH with a four satellite "lock on " ;)

Bang on M8 and Faaaarrrrr more acurate than any speedo that is not calibrated every Week !!!
 

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My bigger wheels and tyres have made my speedo correct! Also getting another 10% better fuel mileage! Result!

All I have to do now is update my sig pic to match... :D
 
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Discussion Starter · #9 ·
My old terrano had a truly hopeless speedo, the XT is much better - 56mph on the speedo on cruise keeps place with wagons on speedlimiters on the motorway, so seems to be pretty accurate.
 
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