Mercury 225 Manual page 135

OUTBOARD INSTALLATION
Water Pressure and Speedometer Hose or Tubing
NOTE: This applies to models without SmartCraft gauges.
This outboard has a speedometer water pickup located in the
leading edge of the gearcase. If you want to use this water
pickup for the speedometer, disconnect the water pickup tubing
from the speedometer sensor and route tubing out of the cowl.
Install the coupler provided with the outboard on the end of the
tubing.
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