For which type, many places like John's Mustang will know what your tranny takes. It depends on which side of the tail shaft the speedometer cable comes out.

As for initial size, install what you have and see how your tach and speedo correspond. More algebra...

RPM=(336*gear ratio*MPH)/tire diam

EDIT: FYI, it's a lot easier to make a spreadsheet with these formulas in it and just change the variables. I have one already madeif you want me to e-mail it.

 

I have just determined the gear ratio at 3.00 and am using the 7 drive gear as a guess. It looks like I need a 16.29 tooth gear for the speedo into the tranny. It enters the tranny on the drivers side so I am guessing it is a rh rear type 3??? Any further suggestions??

 

Wow...they cover quite a range. A good argument to get out of a speeding ticket maybe?
So 60 mph with a 2.70 gear would alsobe 46 mph with a 3.50 gear, same tranny...just doesn't make sense.

 

If the formula comes up with 16.29, I'd go with a 16. You should be a little fast on the speedo, which is better than slow. A rule of thumb is that 1 tooth is about 3 mph. I have tested the thumb rule with a formula, but it's close enough.

69mach...The drive gear is the same, but the driven gear changes to correct the speedo error.