All speedos are not set to be 10% off. Manufacturers (in Europe anyway) have a tolerance range inside which ALL of there speedos must sit. They must NEVER show a speed which is less than the actual speed. The upper limit must not be more than 10% + 5mph of the true speed. And this has to be true for every speedo that they manufacture regardless of the conditions.
Various things affect the speedo reading. Temperature, tyre pressure, how new the tyres are, the profile of the tyre, the load being carried. The manufacturers exaggerate these changes to make sure that they comply with the law.
For example - as the tyre wears down, the diameter of the tyre decreases. As it is smaller, the wheel has to rotate faster to maintain the same actual speed. So your speedo reads higher. Over the life of the tyre, the diameter of the wheel can reduce by about 2%, so the speedo reading will be 2% different from what it was when the tyre was new.
Tyre pressure and increased temperature make the tyre larger. Load effectively squashes the tyre, making the effective diameter smaller.
There are also manufacturing tolerances to take into account. Manufacturers will aim for speedos which sit in the middle of the permitted range. But some will read higher, some will read lower than that middle point.
Satnavs ? Well they can be very accurate, but it depends how you read them. Treat them like a speedometer for a quick glance, and they will not necessarily give the correct reading. Standard satnavs use a distance / time calculation, and the position fix is often subject to error. Mine rarely gets below about 4 yards, one way or the other. Potentially that is an 8 yard error between two position fixes say one second apart. That is quite a big error for speed calculations.
So the satnav will take averages over a number of position fixes. Which take time to calculate. So the satnav never shows you how fast you are going. It shows you how fast you have been going up to this point. And if you do not have a clear view of the sky due to mountains, trees, certain cloud conditions, bridges, tunnels, large artics alongside, changes in speed, ........ the errors will be much greater.
Some satnavs make assumptions about where you are. If you have a route plotted, for example, it will assume that you are on the route, and haven't just deviated. If it loses the signal briefly, it will assume that you are continuing at the same speed. You can observe this if you enter a tunnel slowly and then speed up. The satnav will assume the speed you were travelling as you entered the tunnel, for the entire tunnel length. As you exit and pick up the satellite again, it will briefly report that you are doing (say) 130mph as it catches up with your actual position. (My Zumo 660 does this. My 550 did not).
But given a good view of the sky and a constant speed, a standard satnav is pretty accurate.
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