A 39-tooth gear is from a TH400 and the sizes don't go down to to 19, which is a manual piece.
Street tire diameters don't REALLY vary much from 26", so we'll guess the same as stock.
Each step in rear gear is about 5 percent, so going from 3.08 to 3.73 is 4 jumps: the driveshaft will turn 20 percent faster.
Slowing down the speedo cable by 20 percent takes a driven gear with 8 more teeth: 47. Unfortunately, the biggest gear available is a 45, which by the way takes a different adapter sleeve.
The factory used conversion adapters on the cable to finish the correction. You can get stuff like this from speedo specialists, but I don't know of any off the top of my head.
Of course, if your tire is bigger or littler than stock, it will change the ratio as a percentage as well.
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