^ Why is that? Because I didnt understand where speed was measured for the car? ... I thought it was a decent question to ask... If you make your tire diameter different your speedo reads wrong. I didnt know whether it was measured at the front or the back, so I didnt know which change to accomodate for since they would both be opposite. Isnt that what the forums are for... To learn things about the car?

 

If memory serves, most of the OBD Ford cars measure using a device called a VSS (Vehicle Speed Sensor), which is installed on the drivers side of the transmission. I've replaced one on my son's Contour SVT once because the shaft had broken and it caused a myriad of problems including no speedo and no cruise control. It is not mechanically connected to the speedo, it is a "measure by wire" device feeding the ECM of the car which translates the varying voltage/current in the sensor to the correct speed. When you change the computer's tire size using a tuner, it recalibrates the computer's interpretation of the varying voltage/current of the sensor and recals the speedo to reflect the correct speed.



UB10