I have an 83 z28 with LG4 305 700r4 and 2.73 open rear end. I purchased a used 3.42 drum brake posi off hawks website (hard to find in New Orleans). Im running 20" iroc wheels with 245 tires and my speedo is off about 4mph. With this 3.42 rear end will any other changes need to be done like stall torque converter? With the speedo being 4mph lower with the wheels will the 3.42 will bring speedo to normal? I wanted a posi unit for my future ls1/t56 swap that I keep pushing off. I know they don't hold up well to power. What kind of improvement should I expect to see in performance? Will it be noticeable? Thanks Randy

 

You will feel quicker acceleration. Your speedometer will be off even more than it is now. No need for a torque converter change. You didn't say what year the rear end is from, so it could have either 26 or 28 spline axles in it.


If this is another 3rd gen rear end then the "posi" is probably going to be an Auburn. It may last a while, or it may already be worn out. It's not a good idea to buy a used Auburn, or even a new Auburn as far as that goes. You may find yourself changing the differential in the near future. If you do then do not buy another Auburn. Get an Eaton or a Yukon.

 

Yeah, your speedo will be way off in the opposite direction, and if like mine it will be more than 4 mph and it will change as you go faster. I have 3:42 posi with 19" vette wheels and 35 tire wall.My car was a 2:73 original also. I am about 5 mph faster @ 45mph. At 72 mph I read 80 on the speedo. Your 20" rim may give you a little different reading on the speedo.

 

 

At 72mph (actual speed) around 2600-2700. At 80 mph (actual speed) around 3K

 

If it starts spinning one tire then you know it's shot. Hopefully it will last a long time. Just don't do any burnouts while holding the brake.

 

Thanks guys cant wait.

 

You mean like this>>

 

 

Would you mind terribly saying what Hawks charged for the diff?

I’m swapping a T56 in my car, and need to find an inexpensive 3.42 axle. I can’t afford a 3k new differential at the same time I’m spending for the T56.

 

 

Thanks..I live close to Hawks, and could pick up the differential directly from them. Thanks for the info!

I lived in your area in the mid 80’s, on the Westbank, off of Lapalco, and Wall. I used to be a Kmart manager for the Harvey store on the Westbank Expressway, next to the Harvey Canal.

 

 

You can use this program to figure out you speedo gear. (http://www.bgsoflex.com/speedo1.html ) Then order your gears here; (https://www.transmissioncenter.com/s...4-driven-gear/ )