GPS Useless: Fly Attitude
The aerodynamics of the plane is a perfectly good airspeed indicator.
For a given weight and cg, the pitch angle (nose up/down), as shown on the Attitude Indicator, will indicate airspeed. Pitch down, and the plane speeds up; pitch up and the plane slows down. It's there staring at the pilot throughout every flight.
Oh, that happens to be the basic of the Unreliable Airspeed Indicator training.
Of course, some 35 years ago, the FAA began preaching that pitch controls altitude and power controls speed, and too many bought into it, but that's for another thread..