Ok guys, my rear end just went in the bird. I have a 89 formula with LB9/auto. The rear end in it now is the stock 2.73 non posi. I was thinking about swapping a posi into it. I just cant decide which gear to look for. I mean the 2.73 highway gear is nice because i plan on doing quite a bit of highway driving next fall to college which is 300 miles away, all highway. However I dont know if I should switch to like a 3.10, 3.23, 3.42, or even 3.73. I dont know how much this would effect my highway mileage. And I dont know how much it would cost to switch the speedometer thing for the ratio change. Any insight is appreciated.

 

With that much driving I wouldn't go past the 3.23 gear. You can use an Eaton Posi with the 3.23, but not with the 3.08 or 2.73 because those gears require a 2 series differential. As far as I kow the Eaton is only available in the 3 series. The Auburn is available in a 2 series, but I can't recommend an Auburn. Your '89 should have 26 spline axles, but you can upgrade to 28 spline axles and use a later model Torsen differential.

 

ok so, no one offers the 2.73 posi rear? or the 3.08?. I want to swap out the whole rear including the housing because its all scored, the mesh gears are toasted too. So i am looking for a whole new posi rear. Probably reconditioned or rebuild from a transmission shop.

 

The 2.73 and 3.08 were offered with posi from the factory. They did not have the Eaton Posi, but had either the Auburn or Governor Lock. You can find complete rear ends with these gears and posi if you look for them.

 

Oh alright thanks, i will see what i can find. How much is it to swich the speedo thing so i can still use the stock speedo if i decide to change ratios?

 

There is a lot of information about the speedometer gears in the sticky threads.

 

Yes, but you'll have to change the drive and/or driven gears depending on what ratio you swap to.

JamesC