Under the assumption that the Earth is flat, either my speedometer lies to me, or every time I step into a plane the government modifies my memories. Take your pick.
So fix your goddamn speedometer... make one yourself and rig it to the plane and see if there are discrepancies.
Here are two solutions that would solve your problems with finality;
1) Wind. If the wind blows against you at 50km/h, you're flying much slower despite what your speedometer tells you unless it's a GPS system. Although, you being a upstanding FE'er, you don't use GPS because it lies. So being restricted to an analog speedometer, you'd have to continue checking with stations on the ground to double check the wind direction and speed.
2) You're not flying length, you're flying an arc length... 15% further than what
you believe you're flying since you're assuming you're flying a straight line, when in fact its a long arc.
Please report back with new information.