http://www.apexgarage.com/tech/gear_ratios.shtml

gear ratio must be calculated in decimal

gear ratio is calculated for a lt250r 87 first gear  primary reduction 2.681 x first gear ratio 2.384 = 6.392 this is the first gear ratio, you must calculate each gear ratio using the service manual spec.

final drive  is your chain sprocket ratio in decimal ex: 42/13 = 3.23