I’m sure this has been discussed a few times...

I have an ‘88 L98 IROC-Z, currently swapped to a T5.

I have a freshly rebuilt T56 that I’ve been wanting to put in the car for nearly 3 years now, but a family illness has put it off until now.

Currently, the car has a 2.56 drum brake rear, which someone put under it to replace the original disc brake rear, which is long gone...but they never replaced the proportioning valve, so it still has the disc valve.

I was fortunate enough to find a T56 1386-000-009, used in the ‘94-‘95 Camaro/Firebird on Craigslist for $800 three years ago. I had it rebuilt, but the 2.66 first gear means that I’ll definitely want a much more aggressive rear axle ratio.

I’m aware the 4th gen cars got a 3.42 with the T56, but feel that since 5th and 6th are both overdrives, and I live in the mountains, that something a bit more aggressive would be a better choice, say a 3.73 or so. I will mostly be driving on the street, with maybe occasional visits to a 1/8 mile drag strip, but those visits are probably going to be rare, so I’m looking for a suggestion mainly for cruising, with good performance, and reasonable fuel mileage.

I’m putting the Hawk’s 17x9 IROC wheels on the car, so I’m also looking for a good rear axle of stock width, with Posi, and I’ll want to put rear discs back on the car....but most of the options I’m seeing look like I’ll be spending a ton of money on a rear...so I’m also looking for reasonable cost, if possible for the new rear end.

Suggestions?

 

If you don't want 3.42's, which are really perfect for a street car, then don't bother with the 3.73s, just go for the 4.11's

 

I actually like your your suggestion of the 3.42...because MAYBE I could luck up and find a thirdgen 3.42 posi rear for not too much money.

Hawk’s wants over 3K for a Ford 8.8, and to me, that’s sort of ridiculously high.

 

I have a puny, stock, 4.8, T56 and 3.45's and it's perfect for a street car. Most of the other guys are going to tell you to go with the 3.73 or 4.1s. I can use 6th gear down to about 45-50mph to cruise around town with the 3.45s. As long as I am on flat ground and not trying to accelerate it's great to keep the RPMs down.

Someone on Facebook is selling a NOS set of 3.45s for a 9 bolt

 

Unfortunately, I have a 9-bolt posi disc rear sitting in my back yard, but it’s a 2-series carrier, and IIRC, 3.45 gears won’t physically fit.

 

No indeed, you'd need a 3 series.

 

4.11's with T56; don't even get into 6th unless I'm at at least 75. ( & over 80 when I had 3.42's )

** I simply CAN'T drive a stick @ 1300RPM.

 

As far as that goes, I had 3.73s and a T-56 (with .50 6th); I found 6th altogether useless below about 70 or 75. Pretty tough to drive in traffic at say 60 mph with the motor at 1100 RPM.

I wouldn't recommend 3.45s with a T-56 either. Just commenting previously on the fitment.

 

If only there was a 3.90 gearset available for the 7.5" 10 bolt

 

17s have fewer tires available these days. Check this:

https://www.thirdgen.org/forums/whee...sh-wheels.html

And yes, 3.90 gears.

https://www.strangeengineering.net/p...3-series.html/

Wheel sizing and gear ratios can really change how enjoyable a car is. Choose wisely.

 

My 99' SS has a T56 w/2.66 1st gear and 3.42 rear end. It's fine on the street, but it's not optimal. 1st gear feels a bit tall and 6th is useless below 70MPH. The 350TPI will not have an issue with the 3.42 rear, but if I were choosing a rear gear for that setup, it would be a 3.73.

Now, 3.42 and 3.45 rear gears were much more common than 3.73's in thirdgens and if I came across a good used 3.42, I would just swap it in and run it as is.

 

$3K sounds like a lot until you start adding up everything you need to buy to replace gears/carrier/bearings, maybe axles inside the 9 or 10 bolt rear and then paying someone the labor cost to do the work and set it up. If you do the work yourself (and have the tools and knowledge to set up the gears), it could be cost effective. Figure parts to be around $1K.

 

All these people saying, over and over, that 3.42s are numerically too low must have REDICULOUSLY big cams that make WAY less torque from 12-1500 than my PUNY 4.8, that makes less than ~280 PEAK torque

 

I have a bone stock 04 GTO LS1 M12 6 speed 3.46 gear. Sure the gear ratios are a bit different on a m12 and I personally like a higher geared axle over a low gear but I have zero complaints on 3.46 ratio for the street. I skip 3rd gear most the time so I don't know why I'd want a higher axle ratio.

 

There's a lot of apples to orangutan comparisons possible when you start changing what engine people have, what ratio they have, what they are used to. It is INSANE to me to read other forums where there are guys who feel a 4 speed, no overdrive and 4.10 or higher gears are what they suggest for a street car. Some people used to older cars feel like a car should be in powerband at all times, and never lug at all. I am the opposite and like to feel like the vehicle should be able to poke along and get mileage. Other people are in the middle.

But back to the OP, an L98 and 3.42 isn't ideal with a 2.66 / 0.50 rear. The same with 3.23 is terrible. I never liked 4.11 with that combo because it's more than the engine needs (as a lower-rpm powerband engine) and it takes away from the highway mileage and quietness potential. And 3.73 is so darn close. Once you drive L98s with those different gears, you know what you like.

The M12 ratios are evenly spaced (same % drop 1-2, 2-3, 3-4) and are better in the heavier cars like the GTO. The 1-4 spread is similar to a third gen V8 T5, and would make a nice setup with an L98 and 3.42.

But back to the OP, and L98 and 2.66 / 0.50 . . .

 

While I do understand that, my car weighs ~3600 lbs and I have 11" wide tires on all 4 corners and an engine that makes less power and torque than most of the cars that other people here drive. I don't see how it's possible to not be able to use the 3.42s with a .5 6th gear on the highway at 55-60 unless they have some radical cam. I am not trying to accelerate at 45 mph in 6th, just maintaining speed around town

 

Fair enough

 

Over the years I've concluded you choose rear gear ratio based on how you want 1st gear to behave. You can't screw up freeway cruising with a T56. You just can't.

People should use more gear than they usually think.

 

One of my T56 Transmissions is on the road (rear tires are 295/ 35ZR18)...
It has the gear-set with a 2.66:1 First-Gear and a 0.50:1 Sixth-Gear.

While I completely agree that First gear is usually considered most important when choosing the Differential Gear-ratio.
I however, considered Sixth gear to be the most important...
Being that the T56 is a double-overdrive unit (at least with this gear-set) I chose a 4.30:1 ratio, so I do not have to cruise at a minimum of 80 mph in 6th gear.

 

Biggest issue from that standpoint is I live in a mountainous region, so steep hills are pretty common, even on the Interstate.

 

So in that case you're still going to downshift into 5th, or 4th, while on the interstate to maintain speed,