Rep Power: 755

I'm now considering getting gears for my car. I need some information to help me decide between 3.42's and 3.73's. My main question is how much is my gas mileage going to suffer, and about what RPM am I going to be sitting at when going highway speeds with each one? Keep in mind I have an auto 3.8.

Also, what the hell is the difference between a Posi and a Limited Slip? I have never read that anywhere.

 
Rep Power: 1093

posi and limited slip are the same thing.

I have never gotten my speedo recalibrated so I dont know what rpms 3.42s sit at, otherwise Id help ya out.

 
Rep Power: 783

I cant judge too well considering that im an M5, not A4, but im doing around 2,200rpm @70mph with 3.73's. 3.42's arnt enough of a noticable increase from 3.23's imo.

 
Rep Power: 885

If you commute a lot on the highway then the 3.42's are going to be your best bet, but if you are going to spend the money to swap the gears, you might as well go with the 3.73's. If I remember correctly there was only about 200 rpm difference between the 3.42's and 3.73's at 65 mph- or around 100 kmh for you metric people..

 
Rep Power: 755

I don't do nearly as much highway as I used to so I will probably go 3.73's, and if there is only the 200 rpm difference between the 2 that isn't bad at all.

Will I have to change anything else in the rear end when I do this or can I just swap them and leave the rest stock?

 
Rep Power: 885

Just make sure you get the right gears for the carrier you have, get an install kit and you should be all set. While you are in there might as well get new axle seals and make sure everything else is ok.

 
Rep Power: 1125

yep replace all your bearings and seals as a preventive measure

 
Rep Power: 783

Yep. I'd also try and see if you could at least find a used Posi to swap in there while your at it. But thats just me.

 
Rep Power: 755

The reason why I'm thinking about the gears is cause one of the seals has to be replaced so I figure while it's opened up might as well do the gears and posi. Thanks guys.