Actually, my unit is doing better at speeds (the base unit needed a tune up at the shop - and since getting it back is working nicely). From slow to about 6-7 mph, it's fairly clear. From there up to about 20mph, it is not bad (not clear, but still readable). From about 20mph to maybe 35mph, you can still make out the bottom - but it's difficult. Anything over 35mph is bye-bye.
I mounted the speed wheel for testing and verification purposes. And no, it's not very accurate. Let's face it, that little wheel can only keep track for so long. At lower speeds (maybe up to 20 or so), it's pretty close.
I have a Garmin Handheld GPS with mapping that I use all the time. I have the 981cSI unit, therefore it does not have mapping. It does have the GPS though. With my handheld on a RAM mount with Hot Spot maps, I really don't use the GPS that much. I do use it some with my front mounted Hummingbird 87c. I need to start using it more, to mark creek channels and such that I find with the 981 unit.
I'll do some other documentation, along with pictures, and put up another thread. I think I am close to some type of breakthrough regarding 'interpreting the data'.
Tex