Terry – what is your ” Mac26M built-in depth/speed indicator” connected to? Who makes the device, and what type of a sensor does it use?
On my fishing boat, I have an older Humminbird unit and the speed is determined by a little water-wheel device that sits in the water right next to the transducer and the default reading is rarely accurate, and it gives STW (Speed Through Water). In the Humminbird master settings, there is a +/- setting that allows you to adjust to “Actual” speed which can be measured by Navionics (which uses GPS “SOG” – Speed over Ground) when you are in a no-current environment. Hope this helps.