I had this laying around and thought it might be interesting to some. This is how you calculate your Final Drive Ratio off the starting line.
If you have trans gear ratios of:
1st Gear = 1.31
2nd Gear = 1.38
3rd Gear = 1.00
Rear Gear Ratio of 3.90
You take the gear ratios and multipy them:
{1.31X1.38}=1.81 is your First gear ratio
Then take the first gear ratio times the Rear Gear ratio and that equals your Final Drive Ratio:
1.81 X 3.90 = 7.06
So your Final Drive Ratio is 7.06