I have the Davis Turbo Meter. I purchased this from Defender Marine about 10 years ago, and it's still going strong. I was with a friend, and as we were drving from Defender I held it out the car window. When the car was doing 55, the meter read 60, at 30 it read 35, at 10 it read 15. I was intially disappointed in this steady 5mph "error". Then we stopped, and it read 5 mph! Oh yeah, I guess the wind was 5 mph in our face. We turned around and tried it again, and this time it read 5 mph low...
