Jix01

Ok so i have a 1990 firebird and i haven't took the diff cover off yet but i looked around and it seems that i have 2.73's?i don't know anything about gearing but i wanted to know if 2.73's are good or bad? i have noticed that it is pretty hard to burn off from a dead stop on dry pavement unless i really hammer on it but if i go around a corner it burns off pretty easy and its usually around 1600 rpms at 70 MPH.

Air_Adam

2.73 is a fuel economy gear ratio, not a high-performance ratio. Typical performance gears for these cars are in the 3.42 to 4.10 range. A 2.73 rear end gear will make it harder to burn off the tires because there is less torque multiplication being made at the rear axle, so basically less power getting to the tires. The benefit though, is it keeps the RPMs way down at highway speeds, which helps fuel economy.

Jix01

perfect, thanks alot

89IrocZ350TPI

Id go with 3.73's.

Cadillac

Or compromise with a a 3.27 or 3.23.

Jix01

well im still looking to get good gas mileage as this is my daily driver. not sure how much gearing will effect that.

JamesC

iTrader: (5)

Taller gears mean more RPM and RPM equals more fuel. With gas inching toward $4.00 a gallon....

JamesC

DJP87Z28

If its NOT broke leave it alone. Unless you mod whats up front that gear change is not going to worth it just to burn the tires.

89IrocZ350TPI

Well for performance a 2.73 rear is close to useless. It all depends on what he wants out of the car. For highway driving my 2.77's were great, but they sucked off the line. 3.45 gears made a HUGE difference in pick up, and I still get decent highway. I say if you want a good daily driver go with 3.42's. 3.73's if you plan on hitting up the track. Or just keep the 2.73's. Your call.

Cadillac

There is something to be said about cruising on the interstate at 85 MPH at only 2200 RPM. I also managed 28 MPG one trip after a tune up with the 2.73 "Unlimited Slipper".

Just sayin',

Jix01

well i think il wait till i mod the engine a bit more . but hopefully gas goes down which it probably won't it is 4.08 for regular here lol

Air_Adam

How good it is on gas really depends mostly on where you drive - if its mostly highway, then the 2.73s would be a good gear to keep. If its mostly around town or at low speeds, then 2.73s might actually be hurting gas mileage (engine has to work harder to push the car at low speeds with that gear).

If its mostly around town, changing to a 3.23 or 3.42 gearset would probably improve fuel economy noticeably, as well as performance, but the offset would be that highway fuel economy would suffer somewhat. It would not be a huge drop in highway mileage, but probably would be noticeable.

Jix01

so what kind of car would i be looking for to get 3.27's or 3.23's out of?or can i buy them? which is probably very expensive im guessing?lol

Darkshot

iTrader: (4)

I have a 350 TBI and I swapped from 2.73 to a 3.42 axle from a 2000 Camaro SS. Neither my city or highway mileage changed at all. My car gets 15-16 around town and 27-28 on the highway.

The performance increase was incredible. Felt a bigger difference with the gears than I felt with any other mod.

Cadillac

That's not my experience. I am exclusive, in-town driving and my economy went down. I've had my foot into it a bit more though. It takes off now (well, better than it did before).

Read this: https://www.thirdgen.org/forums/tran...-opinions.html

Nine bolts can be found in a lot of cars but the best ones are supposed to be from the '89's: GTA, TA, some Z28's, IROC's (I think). RPO codes are the best way to tell. Ten bolt cars have the 3.23, 3.42's and 3.73's I think. Some none bolts apparently had a 3.45 too. There are a bunch out there. Avoid the Gov Lock rears. Diff expert to chime in here...

I wish I could say the same for the first part but totally agree with the second.

gcgarner

I let a 3.27 nine-bolt from an 89 formy w/ rear disks sit in my basement for over a year before I sold it for $150.

Wish I had it back.

I'm putting a 350 in now and bought a set of 3.23 gears out of a 4th gen to swap in my old 10 bolt rear.

Can't wait to get rid of my 2.73, but I still want some highway-friendly gears. I think 3.23 is best all-around.

92RS702

I went from 2.73 to 3.23 and didn't notice a big change in mpg on the street or highway,but I did see a nice change in acceleration,so I would have to agree with gcgarner,3.23s are the best all around.

Jix01

alright so whats the axle code for 3.23s?

gcgarner

Did you put the gears in yourself or have a professional do it?

My gears aren't going in until my new motor is in (about 2 weeks). I was wondering what the cost of swapping from 2.73 to 3.23 is going to be. Is it anything more than labor & a rebuild kit?

abyliks

i had 3.73s in my 92 and still got 21-22 on the highway, 3.42s would prob be well rounded for everything

and if you have an open diff your going to want to upgrade to a LSD like auburn or detroit true trac

tylersb350

i have a 2.93 posi. i dont know my fuel mileage. i thinks its gallons per mile not miles per gallon. i dont have a problem burning them off but i guess that 450hp is a part of it. hope to have 3.73 soon.

Cadillac

Sounds perfect for a 450HP car.

My ideal ratio for today's mileage concerns and balanced with performance would be the 3.08 posi (carrier II?) and now the 2.93. Seems a good gear. In the carrier III's, the 3.23 and 3.27.

92RS702

Actually I swapped the entire diff.My old one was really rusty and had a crack on the pan har mount so it was better to swap the whole thing.

Jix01

so still wondering what i could get 3.23's out of?lol

92RS702

Are you looking to get it from the junkyard or buying brand new?

Cadillac

Read the thread OR... consult the site:

Third Gen Tech Data Site for '92 FB.

Jix01

Junk yard hopefully

92RS702

Any 3rd gen that is 90 and up will work,just make sure you check that the lsd still works before you take it apart.Have you thought about swapping in a 4th gen rear,you can get them with better brakes and less miles than a 3rd gen diff,and it will bolt in just fine.

89IrocZ350TPI

Alot of people on here say 2.73 to 3.23 is not worth it. Ive always been told 3.45s are the way to go.

Firebat

iTrader: (7)

In an automatic, most 3rdgen owners will tell you to get 3.23's or 3.42's. Most 4thgen owners will say 3.42s or 3.73s.

Air_Adam

Actually most 4thgen owners will say to START with a 3.73... and most start off with a 4.10 - at least, thats what the 6-spd guys do.

92RS702

For a 3rd gen with an automatic 3.73s,and 4.10s are way to low.For nice performance and decent gas mileage go with 3.23s or even 3.42s,they are very close to one an other,you won't regret it.

Stevo

iTrader: (45)

My gf's 2000 T/A auto had 2.73s, yes rare but it did, also has a steel ds. We just swaped to a 3.42 rear. Her mpg went from 27 highway to 22-23. City mpg stayed the same.

I would go with 3.23s if your concerned with mpg. You WILL notice a big difference in off the line acceleration. When i went from 3.23 to 3.73 i also noticed a big increase of acceleration.

2.73 to 3.23 is a perfect swap for a street car. Automatics of course.