Stuartleah
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I have noticed that when I drive at 50mph on my speedo my Satnav says I am only doing 46mph.
Which is correct??
Which is correct??
I have noticed that when I drive at 50mph on my speedo my Satnav says I am only doing 46mph.
Which is correct??
I find the sat nav so much easier to read and see then the speedo that I now tend to use it even when I know where I am going just for the read out
SatNav. GPS accuracy is significantly better than 1mph at normal speeds. The speedo isn't really deliberately inaccurate, but can't take account of changes in tyre diameter owing to wear and different manufacturers' tyre diameters etc. For example, my Alfa (car!) came originally with the option of 15", 16" or 17" wheels. I don't think the speedo is any different. The required accuracy of a speedometer is -0+10%, plus 4 km/h. That means at an real 60mph your speedo could be reading 60+6+(4*5/8)=68.5mph. That's a long way off.
I understood satnav measures your horizontal speed? If you were going down a hill your horizontal speed could be less than your actual surface speed, or speedometer reading, so an easy target for the speed cameras????
Oh dear...
You overlook something. Your car may have 15, 16 and 17" rims fitted but your tyres have a DIFFERENT aspect ratio. Means your OUTSIDE diameter of your tyre is always the same, no matter how big your rim is.
It's the second number on your tyre which gets smaller when the rims get bigger.
like 205/55/15 becomes 225/45/16 or 215/40/17. All these tyres have the same outside diameter and the speedo (and much more important your gearing) stays the same
I understood satnav measures your horizontal speed? If you were going down a hill your horizontal speed could be less than your actual surface speed, or speedometer reading, so an easy target for the speed cameras????