TSC1 Nautique Top Speed?

Like Tim says, if you are using a GPS based system (Speed control or speedometer) that device will indicate actual speed over land (or in this case water). So if it says you are going 35 mph and you are going upstream against the 5 mph current, you are moving at 35 mph relative to a fixed object located on the earth (such as a slalom buoy).

But if you were to also have system that uses the water to approximate speed (such as a paddle wheel or even an airguide) because the water is actually moving, when you head upstream you would have an indicated speed (on the airguide or paddle wheel system ) of 40 mph and down stream speed would indicate 30 mph. None of this changes the fact that relative to that fixed slalom buoy you are still going 35 mph which is what would be indicated on your GPS based speedometer.

Having said all that, I do not know what the official USA Waterski rules are with respect to slalom speeds in a course on a river.

TRBenj wrote: I know what you meant but you mis-stated an important distinction... GPS speed is never measured over water (which could by its nature, be moving due to current, etc), but always over land.   Think of it as measuring speed over the river (or lake) bottom.

I concur, minor clarification of my parenthetical statement required.   

It's been so long since I have done in any real engineering, that what I think in my head doesn't always come out the same in written form. HA!