A Ford Guy
Join Date: Jun 2001
Location: Lara, Australia
Try a new speedo drive gear.
You may want to try an easier way and change your speedometer drive gear. There are 3 used on the AUII Falcons.
6 Cyl. Sedan Auto's (non LSD) with a 3.08:1 diff have a brown 20 tooth gear
8 Cyl. and 6 Cyl. Auto with a 3.23:1 diff have a light green 21 tooth gear
XR's, both auto and manual have a 3.45:1 diff and use a grey 22 tooth gear
If you go from 20 to 21 teeth that'll decrease your speedo reading by 5%, from 20 to 22 will decrease it by 10% and from 21 to 22 will decrease it by 4.8%. Not exactly your 6.7% that you wanted but it could help.
How are you checking your speedo accuracy? I find the best way is to use a hand held GPS unit that gives a speed readout.