The speedo needle is driven by the gps signal (through some very neat circuitry and a stepper motor). When you turn on the ignition, it will sweep quickly to 80mph and then swing back to 0 where it settles and then the odometer will activate. If a gps signal isn't seen yet, it will flash the odometer section until it does. This might take a few seconds and occasionally you need to move the vehicle so the circuit can sense something happening and move to the next stage.
This project goes back a while. I did a version of this previously (maybe 2 and a half years ago) and used a pulse output speedometer made by Speedhut. But since then some folks told me they didn't like that, weren't interested in it and appealed to me to make a gps unit. Speedhut no longer makes a 3 and 3/8" gps 0 to 80 speedometer, so I had to look elsewhere. Parts from China are less expensive, but they take a while to get here (the speedometer unit took almost 3 months). In the process I learned almost all the gps speedometers sold by others originated in China, were decorated to suit the people marketing the unit (ie they could have been 0 to 35mph, 0 to 60mph, 0 to 80mph...all the way up to 0 to 200mph, different colors, logos, and there could have been all manner of led schemes, gauges scabbed on, etc). Some are made for boats, some for motorcycles, some for home-made airplanes, etc. And in my original project I created a frankenspeedometer...an electronic S-10 speedometer coupled with a vacuum gauge and a tach. You can see it in the decal sticky at the top of this page. It works, but honestly it is too distracting to be practical so I shelved it.
Below you can see the pulse output speedometer (in the OEM case), the frankenspeedometer (which had to be put into a fiberglass case) and some images of the first brass reproduction case I made for the GPS unit.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
