I've had this snip cut and paste into a info word doc I keep on my pc. I can't remember where I got it from but this is what it says....

The standard pinion speedometer gear on a 3.0L GU Manual has 17 teeth and in my case my speedo is reading 6% below the actual speed.

So to calculate the correct gear:
Standard Gear = 17 Teeth
Speedo Error = 6% Below
6% of 17 = 1.02
Number of teeth Required = 16 (17-1)

Of course if your speedo is reading higher then the actual speed then you need to add teeth.