Lowrance speed reading?

[bb243]

I got a Lowrance X135 with the speed sensor option.  When I go full tilt the speed it reports matches my GPS.  when I troll the GPS reports 2.5 and the X135 reports 1.5.  It is pretty much .5 to 1 mph lower than the GPS while trolling. 

Does anyone have any experience with these?  I know I can adjust (calibrate) the unit, but when I do that it reports a much higher speed than the GPS when I give the boat any gas.  I am not sure which to believe...I cannot imagine the SP-BL paddle on the sonar can be that far off and I don't know how much i trust the speed on the GPS. 

I usually troll lures around 2.5 to 3.  Is that pretty good?  I seem to get good action and anything faster makes the lures to hard to handle.  Any thoughts?

Thanks.

[eyedoktr]

Your GPS will always be more accurate than the paddle wheel. The GPS measures SOG (speed over ground) while the paddle wheel just measures an amount of water. You can actually be anchored in a river and the current will make your paddle wheel measure speed.

[bb243]

Thanks, I think you are right on.  I had another GPS in the boat last weekend and both our units agreed.  i guess the speed option on the sonars is worthless for trolling..  Food for thought.