Quote Originally Posted by steveo View Post
that's a cool toy

but my eyes, in a marine application, gps speed isn't really an appropriate input for something like a VSS, especially in the salt or rivers in diverse conditions.

why? because of current.

think about it. gps is speed over ground, pitot tube or paddlewheel is speed over water.

example 1: you have a 5mph ocean current at your back. you're at dead idle in neutral. prop isn't moving. your 'gps vss' reads 5mph. a paddlewheel would read speed over water of ~0mph.

example 2: you are fighting a hell of a storm, head on into current, and some big rollers. you have this thing FLOORED trying to get home. you are NOT winning this battle. gps speed varies between -10mph and +2mph, despite your boat working its ass off and speed over water exceeding 30mph.

of course gps speed is the most useful metric for display speed in a boat since you equate it to travel time.

in the same token, gps speed is the shits for a rolling vehicle because it doesn't compensate for angle of hills (gps speed up a steep hill is as the crow flies, not as the car drives)

will it really matter? probably not, but on a boat i'd way rather have speed-over-water input for both datalogging and speed-related ecm parameters
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.