1. Speedos should not be off by any amount.
2. The tyres on a car may be over- or underinflated, may be worn or new, tracking may be spot on or off. All these influence speedo reading. In fact, manufacturers claim that any tyre fitted to a car must be in a 3% range of original size, because any size exceeding this would cause speedometer error.
3.
Leon Mk1 speedos are (seemingly) electronic, there is no speedo cable. If so, they read speed via ABS sensors (electromagnets) and the speed on the dash has to be the same as indicated in the Climatronic secret menu (it's the same or at best a 2 kph difference).
4. After checking by comparison to a street radar which indicates speed, the dash speedo is spot on when tyres are new and properly inflated, even the slightest difference may cause a 3-5 kph misread.
5. GPS based sat nav systems have a wide error range because they update their position at large time intervals (15, 30 or 60 seconds), while the car moves and follows the road contour (curves, bumps, flat tarmac, then again a bump, slowing down, accelerating etc). The error in regard to position is about 50 meters on a cheap handheld navigator, enough to show you one corner when you're already at the next one.
6. For these reasons, the sat nav on a car running down a road has a larger error possibility compared to a true speedometer and should be trusted less, not more. If
the same sat nav device is brought up in a plane, which flies a straight trajectory, with constant speed and constant altitude, it will read perfectly, because in the timeframe it takes to update the position the speed and trajectory of the plane do not change.
~Nautilus