Forget the whole discussion and lets just assume we are riding bicycles :-) and then the 1 second interval will never deviate very much from being accurate. Of course I also use my GPS when in jets traversing the country at 37000 feet, and then that 1 second is a LONG distance...

I don't know about S&T but I found that Autoroute 2010 works satisfactorily at 150 mph...

Testing it with Streets and Trips at such speed could be quite expensive though!

Not at Bonneville Flats or on the Autobahn or in an airplane if it is running on a Netbook computer, while you are going from point A to point B in any event.

Actually, I found the digital speed readout on Autoroute's GPS pane to be quite useful and easier to keep an eye on than the car's dashboard analogue speedometer.

But on a segment to segment basis both of those examples will be quite accurate for both distance and speed.

Running on the salt flats is dead straight and level so even the total length and speed will be pretty close.

Once the airplane is at cruising altitude it will also spend the majority of the trip going straight and level so the instantaneous (segment) calculations will be bang on. Likely more accurate than a ground vehicle on city streets because the airplane cannot make directional changes in the very short times/distances that the ground vehicle can. The total trip will be out by the significant elevation changes (if the analysis software does not take them into account) and possible circular holding pattern at the ends.

...ken...