Try this, I have used it for decades and it is always close to radar clocks, the only variable that affects accuracy besides airframe drag is propeller brands. Some manufacturers always state their props as having higher pitch than is measured, this means the rpms will be higher with their props and inexperienced pilots are influenced by rpms. For instance, if I put on a prop that has an excellent rip sound then I know I can increase airspeed by going up 2 inches in pitch even though static rpm will always drop and there may not be any prop rip with the higher pitch. A prop that has a nice rip cannot fly faster because it is operating at its max mach speed. In this case the plane would go faster if the prop fell off in a dive. This fact is illustrated by the current RC world speed record holder being (not a jet), but a slope sailplane at over 500mph. Now lets see what kind of arguments this provokes.