All these complicated solutions when there is no problem.It is going to be the same because your still traveling the same distance at the same speed the transmission doesn't know there is a different speedometer so it is sending the same signal.If the signal was different than when I swapped a German motor into a American car with a completely different speedo in it,I would be like 40mph off.Not to mention everyone that has put a Japanese motor in and not switched speedos.
v = 3600/t???? What is that.What is 3600 supposed to be a variable for the distance he may travel,converted to what meters(1610),feet(5280)?It would be V=D/T.Lets not forget gear ratio,tire size and inflation,weather,air density and grade traveled upon,also the fact that he his measuring in kph,etc....so to make a complicated story short your factory speedo is probably not right anyway.But the fact that if you speed sensor from your tranny was that different then it probably wouldn't work anyway.