I plan on puting 3.73 gears into a rear that has 2.73 in it now. Will this work, or will I need a spacer for the gear.

I also want to get a new posi unit but have no clue what will work for my application(just street driving) Auburn, eaton, powertrax, etc....

 

You can use a spacer or get a 2 series 3.73 gear set.
I have an Eaton and it is a very good posi.
If you are only street driving it and not auto-x'ing then you could also go with the Zexel Torsen, and since it is a gear based posi it won't wear out as fast as an Eaton or other clutch style posi.

I had the zexel torsen and it worked awesome on the street, just sucked on the auto-x since I'd have a lot of inside wheel spin when cornering hard and getting on the throttle, so I went with an Eaton.

 

if you are going to get a new posi then you don't need a spacer. just buy a 3 series posi unit and buy your 3.73 gears.

 

I want to do the same thing to my car. So if I have 2.73 ratios and the want to go 3.73, I just get the 2 carrier set and thats all I need to do? Thanks, randy

 

if you just want to change the gears, then yes. if you are doing a posi unit and gear change then just buy the 3 series gears and unit.