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Topic Title: Speedometer or Sat Nav, which is more accurate?
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Created On: 29 November 2011 09:48 PM
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 29 November 2011 09:48 PM
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My Sat Nav (TomTom) gives speed on the display, but this is always about 10% slower than that indicated by the car speedometer e.g. 60 mph on the sat nav is 66 on the speedo. I reason that sat nav position is derived using very accurate clocks and speed is just the rate of change of position. Therefore the sat nav is more accurate than my speedometer and I can go 10% faster than the speedometer indicates and be within the speed limit.

Am I right?
 30 November 2011 01:05 PM
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There's ample argument regarding this often asked question, but the information regarding speedometer accuracy can often depend on the applicable manufacturing directives of each country. In the UK, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from European regulations in specifying that for all actual speeds between 25 mph and 70 mph, the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph. Quite some tolerance.
 01 December 2011 01:13 PM
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The accuracy of the car speedo will depend on the tyres fitted to the car. The rolling circumference of the wheel will vary a bit between brands, and will reduce as the tyre wears down.

So if the speedo is correct with brand new tyres, it will under-read by the time the tyres need replacing.

S P Barker BSc PhD MIET
 01 December 2011 02:25 PM
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Indeed it will, personally I tend to go for the GPS Systems being more accurate as a measure of actual velocity, but you need to be careful as some GPS devices will not take into account any gradients, and will measure only your velocity at the normal tangent of sea level, they also tend to operate on a moving avarage, and off of point to point caluclations rather than taking into account corners etc.

When I was driving for a distribution company, we had GPS calibrated limited fitted to the vehicles, and the tyre tread wear made a difference to these, they where calibrated at worse case with new tyres.

The digital speedo would usually read 117 - 118km/h before the limiter took action to slow you down, this is fitted for a GPS calibrated 70mph or 112km/h, I must add though that this was calibrated by a specialist device, running on an open test feild track in a straight line.

So you can technically exceed what your speedo says is legal by the use of a GPS device, but be wary in poor conditions and non optimal reading conditions.

I have once however seen a case were a speeding ticket was overturned by a GPS log on a satellite navigation unit. (Although I have also known someone get a speeding ticket overturned by going to Millbrook and proving his vehicle was not capable of the alleged speed)
 01 December 2011 07:54 PM
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I am also inclined to go for the sat nav as being the most accurate, but that is a good point about gradients and corners. So perhaps the answer is to normaly use the speedo but periodically calibrate it on a level straight road using the sat nav.

 01 December 2011 08:42 PM
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Is what I usually did in the van when I was on non motorways, pull onto the A19, calibrate up with the Sat Nav at an indicated 100km/h that is actually 60mph.
 30 January 2012 10:31 AM
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I believe that the GPS will give a more accurate speed indication under most conditions.
The main exception is when rounding relatively tight curves or corners. A GPS measures speed by determining the position a short intervals and determining the time taken to move from each position to the next. It assumes that travel from one measuring point to the next is in a straight line.
This is a fair approximation under most conditions but can be misleading, especialy if taking a roundabout at speed and then returning in the same or a similar direction from which one approached.

I have compared a GPS unit to the speedometer on a railway locomotive and found very close agreement indeed, much better than with a road vehicle.
I believe that this is due to the fact that that trains take only very gradual curves, and that the wheels although subject to some wear , are of more nearly constant diameter than rubber tyres.

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