Hi Tom,
Super Turbine 300 transmission with a 3.08 gears would need a (39 teeth) brown speedo gear. Part # 1359273. They should be readily available at a Buick dealership or possibly an auto parts store.
Somewhere there is information that a black (40 teeth) speedo gear is correct. However, the three transmissions that I've had worked on, have had the brown (39 teeth) gear. I put a 40 teeth gear in my convertible once after reading that info. In a short time that 40 teeth gear got chewed up. Went back to 39 teeth gear and all is fine.
John Egel