Speed winch gear ratio for 6.5hp engine

So me and my friend have been working on building our own speed winch for a while now. We are using a 6.5hp engine with a 9 tooth centrifugal clutch. We have already tried two gear ratios. We used a 30 tooth sprocket on the axle to start but the max speed was less then we hoped. We then tried a 16 tooth sprocket, but the engine didn't have enough power to accelerate us fast enough to be of use. To save us having to buy more parts, and modify are winch more than whats needed, does anyone know what the optimal gear ratio would be?