speedo help for th400

I have a 1972 monte carlo with factory th400. I just switched from a 2:73 to a 3:42 gear in the rearend and I am showing a faster than actual speed on the speedo. Does anyone know how to figure out what speedo gear to use in the trans or where to get one? thanks for any help you may give.

Check out this link. Should help you. http://tciauto.com/tech_info/speedo_gears.htm
You can also go to ebay motors. Type in speedometer gear ratio adapter, and listings for ratio adapters will come up. It just converts it up or down by a percentage.

thanks for the info