Yes, the electronic speedos can be driven by either a hall effect sensor or a GPS antenna (there is actually a conversion in the antenna that sends pulses to the speedo where they are counted; so the speedo doesn't know the difference). Since I didn't notice a good place to mount the hall effect sensor, my mount for the hall effect sensor was a failure. It must be very rigid to maintain a constant air gap between the sensor and the protrusions it's detecting during bumps. After I gave up on the hall effect sensor, I found a better place to mount it on my NG3. I don't know if there are similarly good places on other Renault trannies. Another problem I had, although I really didn't get that far, was that nobody could authoritatively tell the air gap between the sensor and the magnets I glued to my CV joint. The GPS antenna was a quick and simple, if more expensive, solution. The antenna was set to send a certain number of pulses per mile and I only needed to tell the speedo what that number was. It was much easier than driving a measured mile and counting the pulses and then telling the speedo that number - although, as I said, I had other problems so this really isn't that onerous.
One thing to keep in mind is that the more pulses the hall effect sends per mile, the better. In my case, the CV joint seemed to be the most reasonable thing for the hall effect sensor to count from and it had three lobes to the housing, meaning three pulses per revolution of the half shaft which ended up in the low, but acceptable range of pulses per mile. It would have been better to count more pulses per rev. The hall effect sensor can detect bolt heads or other protrusions. Magnetic protrusions are better.
In the end, I don't regret going the GPS route. It is not susceptible at all to vibrations or road hazards such as gravel though if it is mounted properly, that shouldn't be a problem. It does have some disadvantages other than price, but for me, they are minor. It takes about a mile to acquire satellites and that is extended in the case of heavy tree cover or tunnels. Trees are my only concern but not where I live. For folks going through the Chunnel, the trip will just be that much "shorter"!
Here's a picture of mine. I had to request the half round bezel as an angle bezel was standard.
