When we give the speed of a car what frame of reference are we using?

fixed frame of reference on Earth

Explanation:

When speaking in daily life or in a written problem, in general we assume a fixed frame of reference on Earth, so that the speeds of objects are referred to Earth.

In some cases we talk about other frames of reference, but these are explicitly written.

It should be mentioned that all the reference frames are inertial, that is, with zero acceleration, in order to comply with Newton's second law

Frame of reference, in simplest terms, describes the state of motion of the observer. The frame of reference may also be described by using a set of coordinates, time and motion. We formulate all our equations and solve them using the frame of reference.