Fred, I think to test that speed change, you need to do a speed test at a given rpm, remove the gauges and repeat the speed test at that exact same rpm then repeat this procedure at least three times, preferably five or more.
It's dang difficult to measure speed changes as a result of changing something on the airframe. If you don't change anything but land then take off and do the speed test again, it's hard to get the rpm set to give you the exact same airspeed. Even the change in air temperature during the interval makes a bit of difference in the readings. Ideally, these tests would be done back to back. It'll take a bunch of repetitions to overcome the experimental difficulty. Somewhere on the Web is probably a calculator that'll tell the ounces (or pounds) of drag of eight inches of 1/2" tubing at 100 mph. My general estimator brain-cells just can't believe that small amount of tubing is enough to slow an aircraft down by two miles per hour at cruise speed. Ed
