This actually gave me more reason to not like these add-on GPS units for a typical conversion.
In the Datsun S30 world people are swarming to GPS based speedos, because it's "difficult" to create a proper VSS signal, and then calibrating, oh man that takes "forever". :facepalm:
My main problem with something like this is that any GPS unit I have used, personal navigation, smart phones, a couple laptop dongles all took a considerable amount of time to find satellites, even on clear days. There were days I would drive several miles before a good enough GPS signal was found. Now these were positioning based systems, but AFAIK the speed only based systems need the same 3 satellites minimum to provide an accurate speed. Even though I may not necessarily use my speedo as intended, I do like to know just what speed I am traveling at whenever the vehicle is moving. I can see overcast and story days being an issue as well. There are also areas where there are structures, like tunnels that could cause the signals to be lost.
I have yet to have any real issue re-creating a proper VSS input for an ECM or cruise control in several swaps, mechanically. Yeah it may have taken a while to sort out the wheels and sensor location but I know it works, and it's in-expensive to assemble.
Don't get me wrong, I do like the idea of a GPS speed sender, more as a back up, or being able to provide positioning data than just speed, but I just can't justify the prices I have seen and the known GPS issues I have experienced and read about other people experiencing. It's good to hear some positive reviews of something that works, for when the need arises.
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