Well basically the way the speedo circuit works is it allows the ecu to know how fast your moving. When you push in the clutch the motor goes from having load to not have any load at all. The ECU will right away cut fuel to a percentage and slowly idle the motor down as you slow down. Why? Probably Because the engineers dont want the RPM's to drop to idle and have your foot slip off the clutch. Its prolly a damage safety design, i dont know for sure.

Without the Speedo sensor the ecu cuts fuel too much when you push in the cluth and the RPM's drop to rapidly causing a stall situation.

Now of course there are more factors that the ECU uses to determine all of this like TPS and MAP but it kind of gives you an idea, and i am sure there is a lot more to it than just that simple way of looking at it.


Terry