The speedo is the legal requirement not the sat nav. Before 2001 speedos had to be within + or - 10% at 30 MPH. That is why all speed cameras are set at +10% +2 MPH. As from 2001 speedos cannot indicate slower than the actual road speed. But they can indicate 10% faster than actual road speed. So at any indicated speed the actual road speed can be lower than indicated but never higher. Tyre deflection with load, tyre wear reduce the rolling radius of the wheel so that is the reason for the allowance below indicated. On a vehicle made in or after 2001 fitting larger wheels and tyres that increase the rolling radius makes the vehicle illegal to drive on roads within Europe, because that will cause the speedo to indicate a slower speed than actual road speed..
Some vehicles have to have a speedometer that is within 2%, calibrated before use when new, tested every 2 years, and recalibrated every 6 years. The speedos on these vehicles read the same as my ancient tomtom one new edition.
 
Some vehicles have to have a speedometer that is within 2%, calibrated before use when new, tested every 2 years, and recalibrated every 6 years. The speedos on these vehicles read the same as my ancient tomtom one new edition.
What vehicles are these. Police vehicles?
 
I have my satnav set up to give audible warnings at 105% of the speed limit. Lady voice says 'You are exceeding the speed limit" and it is spot on with what the dial says on a truck's calibrated speedo. To me that would indicate satnavs are quite accurate on speed calculations. My girlfriend hates using it as the lady voice speaks to her far more often...:oops:
 
From memory satnav speed calculations are based on the dopler shift principle. This should be very accurate to 1 or 2 % tolerance.
My Discovery 3 speedo reads 1 Mph faster than my satnav speed indication is showing at 70. Suggesting that the modern LR speedo is calibrated very accurately indeed.
The altitude displayed on my nav isn't always accurate as yesterday, on the M27 it showed the car at -12ft for a few minutes. I know the Discovery can wade but I'm sure it will struggle in 12ft of water!
 
I'm reading this post from the sidelines. My TomTom also reads about 5mph slower than the speedo on my 2002 TD4,fitted with the standard LR 16" wheels, at 70 mph on the speedo. The F/L has the factory option of 15 or 16" wheels and I am sure that LR do not have specific speedo's on the production line for either option. Maybe they play it safe and fit a standard speedo designed to read "Slow".
If I am using the satnav, I tend to take it's speed reading as gospel.
 
In every car I have ever owned, the speedo has indicated faster than the actual speed, measured by GPS. I think this is done to assist in not getting a speeding ticket !
 

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