GPS accuracy?

[David_L6]

I learn more from the tach in reality.

Tim

I don't doubt that at all but I never could find a tach that would read correctly on my Mercs (mags).

Back when I could test on the lake near home I used a speedometer and a stop watch. I have a stop watch holder that tapes to steering wheel and I made sure I mounted the speedo where I could easily see it. The lake here has pilings for channel markers so I always had reference points that didn't move. Only thing that was sometimes tricky was reading and/or re-setting the stop watch without coming back in to pits.