Possibly Exchanging 3.5 to 3.0 Gears on 9" Rear End

[copb8 97]

I'm considering moving to 3.0 gears from my current 3.5 to improve my highway cruise speed a bit. Making this change would mean I could cruise at 80 mph at the same RPM's I current do at 70 mph. I know I'll lose some pop off the line but I'm not really drag racing the car anyway.

I've been looking around for a used 3.0 with limited slip and came across this ebay ad: 3.0 Posi Rear 

What are your guys thoughts on this item. Says it's out of a truck, does that matter? Is there anything in the pictures that makes you concerned? Can I assume 31 spline is ok for my car?

 

[Grabber70Mach 107]

9" is a 9" doesn't matter what it came out of 31 spline is fine and a stronger setup.  What does matter is if you are now running a 28 spline or 31 spline axle.  If 28 you will have to source or buy new 31 spline axles.

http://www.metropartsmarket.com/rearend/measure.html

[copb8 97]

11 minutes ago, Grabber70Mach said:

9" is a 9" doesn't matter what it came out of.  31 spline is fine a a stronger setup.  What does matter is if you are now running a 28 spline or 31 spline axle.  If 28 you will have to source or buy new 31 spline axles.

http://www.metropartsmarket.com/rearend/measure.html

Thank you. This is very useful. 

Off the top of your head, do you know if you can see enough of the end of the axle just by removing the wheel and brake drum? Or is the drum brake backing plate blocking everything? 

[Grabber70Mach 107]

Should just have to remove the rim if it has a center cap to see the end of the axle.  If it has a hubcap or rim w/o a center cap then you need to remove the hubcap and just look at it.

[lanky 44]

I have 2.7 or 2.8 (can't remember) gears and I like them.

[Vicfreg 771]

That housing is a good one, looks very similar to the "N" (nodular) housings, which are really strong.   You will need to buy 31 spine axles.  I got my 9" center section and axles from Quick Performance in Iowa.   Great people to deal with.   https://www.quickperformance.com/Ford-9-Inch-9-Ford_c_37.html

With respect to seeing the end of the axle, since you need to get new axles anyway, not sure that it matters.   When you pull the wheel off, you will see the hub.   You will need to pull the brakes, backing plates and bearing retainer off to get the axles out.

I have attached some pics of my setup, which is a 9" - 3.25 posi with a nodular housing and Daytona pinion.  I use a "big tube" axle assembly (like on Torino's) that has the pressed on roller bearings.   I have attached some pics of the axles, which have the lengths written on them. (different drivers side and passenger side).  I also have a picture of the dimension from my mounting plate to the outer hub flange.  For me, that was important, as I used rear disc brakes.  Hope this helps.

I had a factory 390 - 4 speed Mustang with 3.00 gears years ago, and it was a great highway cruiser.

 

[copb8 97]

Pulled the rear wheel today and found I have 28 spline rear axles. Thanks for helping me not make a bad purchase.

[barnett468 418]

Not all 9" diffs are the same by any means.