Is the stock rear in a 69 Mach 1 with 3.00:1 gear ratio a 8" or 9" rear? I want to add traction lok/posi and a higher gear ratio (3.25 or higher). I want it to be quicker but still want to be able to drive at 70 without the motor screaming. Any suggestions please???
Tired of the one wheel burnouts!

 

It should be a 9 inch....You can easily check by trying to put a socket on the lower nuts holding the third member in..If you can get a socket straight in on the nut it is an 8 inch..If you can't its a 9...

 

Here is a link with pics to what I was referring too....
http://www.kevinstang.com/Ninecase.htm

You can't really increase your bottom end performance much over the 3.25s and still be able to cruise nicely at 70 mph...It's always going to be a compromise ...The only way to have your cake and eat it too is to run a transmission with overdrive...Then you can run a low gear like a 370-4.11 and still get decent mileage and low rpms at highway speeds..

 

That's some good info on the Ford 9" and 8" rears.

 

With a 3.50:1 ratio on mine, the motor is not screaming at 70. At 80 it's about 3600 rpm.
An overdrive would nice, but the snowball effect would probably have me with 4.11s...

 

With 26 inch tall tires and a 3.50 gear you are still revving about 3200 rpms which is more than most people like on anything other than a very short trip...An overdrive would bring that down into the low or mid 2000 rpm range...I sure wish I had one..

 

With a redline near 6000 rpm, I'm not very concerned about anything less than 4000 for less than 20 minutes into town, but yeah, an OD would be nice. If only my FMX would fail more than just leak a little...

 

Thanx for clearing this up for me guys. Frdnut the illistrations were very helpful. So just to confirm. My Mach had a nine inch 31 spline rear ?. I'm going to go with the 3.25 or maybe 3.50'S. Any suggestions on where to by the rear? Brands? Ford or??? hanx again guys.

 

Srry, I did leave out that we have a hump so I believe its the 9"

 

Probably 28 splines. I think we have the same axle code.
What is your engine code?
IIRC, only 428s got 31 splines.