As stated the factory clutches will already overdrive. When a sled is using its overdrive to gain top speed mph its loosing efficiency rapidly with declining track hp as the belt speeds go up. You would only gain mph on a sled, by using overdrive, if it is already geared too low for top end speed when the clutches are in a 1:1 ratio.
Overdrive can be a compromise to a situation when using lower gears for quick acceleration but still maintain a descent top speed but its not a free ride.
Ski Doo has always had their clutches designed with a pretty big overdrive to allow lower gears to keep the sled feeling responsive but notice most Ski Doos don't have long legs.
In the Dragy I posted earlier my sled was geared for @ 102mph at 1:1 and ran up to 112mph using its overdrive but you can see it didn't gain much speed after 660' like higher gearing would have delivered.
JM.02c
Overdrive can be a compromise to a situation when using lower gears for quick acceleration but still maintain a descent top speed but its not a free ride.
Ski Doo has always had their clutches designed with a pretty big overdrive to allow lower gears to keep the sled feeling responsive but notice most Ski Doos don't have long legs.
In the Dragy I posted earlier my sled was geared for @ 102mph at 1:1 and ran up to 112mph using its overdrive but you can see it didn't gain much speed after 660' like higher gearing would have delivered.
JM.02c