speedo gear for a 3.42 rear gear?

Hello, can anyone out there tell me the correct number of teeth at the end of my speedometer cable gear to give the correct reading at my speedometer?I installed a 3.42 gear in the rear end,I am using a t-10 4-speed transmission in my 70 chevelle 454.right now i am about 12-15 mph. off.(going 55mph ,speedometer is reading approx 42mph) Where can I buy the correct one?Thanks

Try this site: http://chevelleengineer.home.comcast.net/speedo.htm

Seems to answer your question.
highlandlake