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The CfA digital speedometer was a spectrograph developed in the 1980s as a Center for Astrophysics (CfA) project, for the purpose of radial velocity (RV) measurements, to pin down the RV component of proper motions of stars, to detect spectroscopic binaries, and it was eventually used in attempts to detect extra-solar planets. It was digital in that it used a photon counter to collect digital data rather than photographic plates. At least three units were built and installed on research telescopes. They detected radial velocities with a resolution on the order of a kilometer per second. Considerable post-processing was used, evaluating matches between a spectrum observation versus calculated spectra based upon a range of radial velocities.