I have a 66 stang i have built a 347 for and have put a t-5 out of a 86 gt and 411 gears in the 8in rearend my front tires are 205-65-15 the rear 255-60-15 question is what speedo gear should i use i still have the 3 spd gear in it which as you can guess its way off so i sure could use some help on this one THANKS  

It took a bit of Googling, but it looks like for a 4.11 rear you would need a 6 tooth drive gear on the output shaft and a 20 tooth speedo driven gear on the cable. If you have a 7 tooth drive gear on the T5, then a 21 will be close but off.  I think 21 is the highest tooth count available.  

Some data here:
http://zenseeker.net/4x4/Speedo.htm
And here:
http://www.fordracingparts.com/download/charts/129.pdf  

thanks