Posting this for Alan with the 70 GTX. He is a member, but not forum enabled. I suspect his speedo calibration is wrong - can't see what else it could be?
"We recently fitted a new speedo cable, which has been over reading by about 20mph. We then changed the Speedo Pinion Gear in the auto box to a 32 teeth from the 34 teeth gear that was in there, but the speedo still over reads by approx 50%. I wondered if you could confirm if we have the right gear. According to the chart in the Year One catalogue: tyre size 235/60 - 15 with a 3.23:1 rear end should require a 32 teeth black pinion gear, which is what we've installed. The speedo cable is new and the speedo re-calibrated."
32 teeth is correct. There must be something else wrong.
It is engaging in the drivechaft correctly isn't it?
The bit that holds the gear wheel is cam shaped and has to be turned to engage the trans output shaft. It makes up for the fact the drive gears are diferent diameters.
Maybe it is slipping?
There is another possible cause. The number moulded into the gear might not relate to the number of teeth. Last year I had reason to doubt a speedo gear marked 34 really had 34 teeth, I counted them and it didn't. Can't remember now if it had more or less. Just a thought.
)
Been there and done it 
