Works well

I first tested out the speedometer function while my wife was driving. The reading matched the cars speedometer. The altimeter function looks accurate.
Then used it to see how fast my little 3.5hp outboard pushed the Jon boat.
The only bad thing is that you need to have data service for it to function so its not like a GPS. So it cant be used in remote locations.

RMW Calgary about Speedometer Speed Box, v5.1