Speedometer Gear Question

So I currently have a stock 2.89 gear ratio in my 78 Ford F150 and I am putting in a 3.73 gear ratio with 29 inch tires so for my speedometer to work correctly how do I determine the teeth count I will need to install back into my c6 transmission????

http://www.explorerforum.com/forums/showthread.php?t=158739 I believe these formulas apply to C-6 also.

http://www.classic-cougar.com/tools/speedocalc/speedocalc.php Try this one, the first one would not link.