Triumph Repair: Spitfire Speedometer, speedometer, pinions

Question
Jim,
I read your answer to the 80mph speedometer question.  (I have a 1977 spit with a 120mph speedometer)  You mentioned at the end of your article that a different differential would result in an inaccurate speedometer reading.  I am replacing my 3.89 diff with a 3.63 diff, and I was wondering if you knew what needed to be done to make the speedometer read correctly?  Change out the speedometer or the drive gear in the transmission?  Thanks for your help!  Warren

Answer
Warren,

I'm not sure if there is a late style speedometer with the correct gearing, but I have found a parts listing for the single rail gearbox (appropriate to your year Spitfire) that does list different drive pinions for the 3.63 and the 3.89.

The factory part # for the 3.63 pinion seems to be AEU1578 and the part # for the 3.89 is NKC49.

The 3.63 is listed as the "red" pinion and the 3.89 listing gives "18 teeth" in the description.

Cheers,

Jim