i did the math to figure out what drive gear i needed to get my speedo correct and it came out to 15.322 teeth. i can only find 16 tooth gears. will this .688 difference make a big difference in the correct speed?

 

Don't know do the math.

 

Sounds like you're talking about the driven gear. It's a lot easier to change than the drive gear. Here's a web page that might help:

http://429mustangcougarinfo.50megs.com/speedo_gears.htm

As it says, you're allowed ~ 3.75% of error so you should be fine with the 16 tooth gear.

Justin