mine is out by 5%. found if you tweak your tyre sizes you can get it back closer to what the actual speed is.
the thing is the car value is given by how many kilometres you have done in it. servicing is at a designated mileage readings
not to mention the km / tank of fuel. switch to the proper size, rpm goes down, fuel use goes down, more km / tank, lowers running costs and the value of the car doesnt go down as fast