I am thinking of changing my diff to a 3.45 LSD from a EL ghia.How do i calibrate the speedo with the ratio change?

Simply change the gear on the speedo sensor.

Sensor is found in the extension housing of the transmission, on the drivers side. Removed via an 8mm spanner and the gear is clipped onto the sensor. Once unclipped, it slides off.

Ford still sells all the different speedo gears needed, and they are between $8 and $11, depending which dealer you vist.

If you have 15" or 17" wheels, you will need the grey coloured gear (22 teeth). If you have 16" wheels, get the black coloured gear (23 teeth).