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In 1996 Mercury changed the gear ratio in the lower unit to achieve a stronger gear set. Prior to 1996 the lower unit utilized 17 tooth and 28 tooth gears. In 1996 this was changed to a 13 tooth and 21 tooth gear set. SEI uses the newer style gears in our lower units. This changed the overall ratio of the drives when added to the ratio of the upper unit. The new ratios can be substituted for the old ratios with no discernible performance changes. The following chart shows the old and new overall ratios.
1995 and older (22/20 tooth upper, 17/28 tooth lower) 1.5 ratio
1996 and newer (22/20 tooth upper, 13/21 tooth lower) 1.47 ratio
1995 and older (24/24 tooth upper, 17/28 tooth lower) 1.65 ratio
1996 and newer (24/24 tooth upper, 13/21 tooth lower) 1.62 ratio
1995 and older (17/19 tooth upper, 17/28 tooth lower) 1.84 ratio
1996 and newer (17/19 tooth upper, 13/21 tooth lower) 1.81 ratio
1995 and older (20/24 tooth upper, 17/28 tooth lower) 1.98 ratio
1996 and newer (20/24 tooth upper, 13/21 tooth lower) 1.94 ratio
1995 and older (22/20 tooth upper, 17/28 tooth lower) 1.5 ratio
1996 and newer (22/20 tooth upper, 13/21 tooth lower) 1.47 ratio
1995 and older (24/24 tooth upper, 17/28 tooth lower) 1.65 ratio
1996 and newer (24/24 tooth upper, 13/21 tooth lower) 1.62 ratio
1995 and older (17/19 tooth upper, 17/28 tooth lower) 1.84 ratio
1996 and newer (17/19 tooth upper, 13/21 tooth lower) 1.81 ratio
1995 and older (20/24 tooth upper, 17/28 tooth lower) 1.98 ratio
1996 and newer (20/24 tooth upper, 13/21 tooth lower) 1.94 ratio