More info. The std P6B speedo for MPH is 1248 turns (of cable) per mile, a KPH speedo is 780 turns/km.
Based on std tyre dimensions of 185-70R14 these should do 518 revs/km, which is 1597 revs of the prop shaft. At those revs of the engine in top gear the speed is 60 kph. If the speedo takes 780rev/km, then the gearbox reduction is 1597/780 = 2.04, so reduction is most likely 2:1.
With my car and its LT77, with the 22 tooth red speedo gear, I wanted to know what the output shaft driving gear was - 7 or 8 lead. My tyres are 195-75R14, 5% bigger than std, which should do 491 rev/km=1513 revs of the propshaft. Reducing these turns by 8/22 speedo gears gives 550revs. 550/780 gives .705, which means at 60kph an indicated speed of 42.3, very close to what I saw on the road. So I think the speedo driving gear is a black 8 lead type. Now that I have a red 20 tooth speedo driven gear fitted I should see 46.5 indicated at a true 60kph. Road test time.......