[thaumasiotes]

Well, the speed of light was measured in traditional meters and then new meters were redefined based on the existing speed of light. So the difference from 40000km attributable to redefinition would be on the order of centimeters or smaller; any deviation could only arise from our inability to measure the speed of light precisely, and we can be quite precise.

There's some difference (~7km, if we believe wikipedia) attributable to the original measurement of Earth's circumference being off, and much much more difference (~67km, ten times as much) attributable to the fact that the earth is slightly oblate. (In other words, meters were defined by reference to the polar circumference, but the equatorial circumference is larger than the polar circumference is.)