The paddle wheel speed accessory will not be as accurate as GPS speed...(in realizing actual boat progression on the water)...

The paddle wheel speed indication is a direct result of water movement turning the paddle wheel at a certain number of "revolutions per second"....

It is entirely possible to be facing "into a 3mph water current" (such as in a river) and the not actually making any forward boat progression .....in this case the paddle wheel would be indicating a speed of 3mph...whereas the GPS would be indicating a speed of zero...

The paddle wheel accessory is utilized best by trollers in moving water because trollers want to calculate their bait's trolling depths as the moving water (such as a river) creates friction against the baits to affect how deep the baits will troll thru the water column....

Stick with the GPS speed to get the best accuracy for "actual boat progression" on the water (regardless of water current effects)...