On a turbo 400 transmission while using a 3.08 rear end on a 70-72 GS... What COLOR of gear did Buick use for the Speedometer Gear for the turbo 400 from the factory and what tooth count? I can find info on the tooth count but would like to know what COLOR the plastic gear was for this combo. Thanks, Eric
Eric, This is a page from a 72 Buick Chassis manual. Apparently, the answer is blue(38) or red(37) depending on what tires you use. If you really want to zero in on the correct gear, you need to know your axle ratio, tire diameter, and the number of teeth on the speedo drive gear. The speedo drive gear is on the output shaft and can be seen when you remove the speedo gear and sleeve. If you rotate the driveshaft by hand with the speedo sleeve out, you can count the number of teeth. It's usually 16, 17, or 18 teeth. Once you know that it's simple. divide the tire diameter into 20168. Take the result, and multiply it by the axle ratio(don't forget the decimal point-3.08), and the drive gear tooth count. Then divide the result by 1001. Round to the nearest whole number. For example, my Stage1 has 255/60-15's which are 27.1" in diameter. It has a 3.42 axle, and 17 tooth drive gear. 20168 divided by 27.1"=744.21 744.21 X 3.42 X 17 =43268.37. 43268.37 divided by 1001 =43.23. I use a 43 tooth driven gear.
You also need to verify the gear you want to use is listed on the speedo housing. I believe one is 36 37 38 39 40 this means you can use those gears only. You can not use a 41 in the housing. If you do, you will ruin the gears. If you need a 41 and don't have the housing, you need to find the correct housing. Most trans shops can get them for you or have them in stock.
