I bought a 998 brand new a couple months ago. Before that I had a 997. And 957. Also a terrova with I pilot. My 997 957 and ipilot. All read the same speeds while I was trolling along. Within .1 or .2 mph. But now I got my 998 its off about .5 mph on the slow side. So when I'm trolling less than 1 mph according to my I pilot control. it says I'm doing .4 -.5 on my 998. Leaving me a big fat donut!!!! I am obviously moving 1mph. I troll often and can tell by line angle and bends in the rod approx how fast I am moving. And I asked hummingbird and they basically told me I was out of luck Cuz they only guarantee to be within .5 mph. Any of you guys have any ideas. Very frustrating!!!!!!!!