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I've never seen a GPS speedometer, not a real gauge type .. I have a old clarion auto pc head unit that has an app for direction and speed (via GPS).. and my ODBII to USB reader can show one on my laptop ..
maybe one of those in-dash screens, an on-board computer and the ODBII/USB reader would do it for you .. but then you'd have a flat-lined stock gauge ..
or maybe the Mad_scientist in you could figure out a circuit to drive the stock gauge ..
I'm not too worried about the old gauges as I'm doing away with the factory dash all together. I could use a relocation kit for the VSS. Didn't think about that for the speedo though. was just thinking about it for the ECM.
found out that the GPS speedos (yes, actual gauge type) are out there (never knew) quite popular with the boating crowd .. but they do have issues with no signal, typical of car GPS when the signal is blocked .. still think the OBD is the way to go if you can as it would be more reliable ..