# What does a speedometer measure?

When speaking of the velocity of a particle, we must specify a reference point. The same holds when we talk about the speed of a particle (is this correct?). If so, a speedometer measures instaneous speed relative to what reference point?

Also, suppose we have car particle moving a circle at constant angular velocity. Note that the both the speed recorded by the speedometer AND the speed of the car relative to the center of the circle is constant.

Now instead suppose we have a car moving along a parabolic path (Ex: y=x^2) such that the speedometer records a constant speed. In this case the speed of the car relative to the origin is not constant.

It seems that the speedometer does not seem to measure speed relative to any fixed reference point, instead it measures derivative of the arc length traversed on the path with respect to time. Is this conclusion correct? If so does this means that speeds can be absolute, meaning they are the same not depending on the reference point?