For the cost of the new carrier and having to get it rebuilt correctly professionally, that cost would be really close to the total cost of purchasing a 5 speed conversion kit from Silver Sport Transmission.
Silver Sport guarantees there are no tunnel modifications, and he would have the choice of overdrive gears.
The OP could retain his quick light to light acceleration and could reduce his highway cruising rpm’s by 900 rpm.
Since he already has a manual transmission the conversion would be simple and something he can do himself in his garage.
He would also end up with a new transmission, clutch, pressure plate, throw out bearing, balanced drive shaft, transmission mount and u joints all included.
This seams like the best option.

 

If you buy the whole differential and its in good shape and can still be hit or miss with the gears inside. Thats what id did with all my rear diffs, jusst easier if its all set up. I had a 3.,55 and its not exactly a tire spinner. You have to run at higher rpm off the line to keep from stalling but it is fine for cruising. The 3.70 is better for both and you can still drive it on the highway. Yes I agree the 4.11 is a bit of a low gear ratio for the highway but it is the best for off the line fun. I went from M21 with the 3.55 and 3.70, then to an m20 with the 3.70 which has a lower first gear which made it fun. I would go with the 3.55 for what you describe and you can always find a 3.70 later, but they are harder to come buy. If you plan on going to a 5 speed save the 4.11 under the bench. If not sell it on here, with a crate it will be pricey to ship thou

 

The OP has a wide ratio M20 trans. So with a 355 it will be fun.
A CR M21 box needs a 411 to have the exact same 1st gear ratio overall.
2.20*4.11= 9.1
2.54*3.55= 9.1

But since he currently has 411s:
2.54*4.11=10.4
That is pretty deep.

How big is the cam duration?
That comes into play here also.

It is the close ratio boxes and their frequently used factory minimum 3.70s that are kind of soft/slow off the line.
2.20*3.70=8.1 overall
Those really benefit from 411s. Or even better, a wide ratio box.

Wide ratio boxes are just a little more rear gear tolerant.

 

That's a good point when discussing the Muncie 4-speed close-ratio. Corvette buyers and owners at the time complained, sometimes bitterly, about the 2.20:1 first gear ratio in the Muncie close ratio gearbox as it was sluggish off the line, as also pointed out by Leigh1322.

However, it should be noted that during the '74 C3 model year, Chevrolet changed from the Muncie to the Borg Warner Super T-10 4-speed and with that change first gear for the close-ratio version went from 2.20 to a deeper 2.43. This change was warmly received at the time.

Therefore, in the way of comparison, the math looks like this.

'68-'74 Muncie close ratio first gear: 2.20*3.70=8.1 overall
'74-'79 BW ST10 close ratio first gear: 2.43*3.70=9.0 overall

The wide ratio first gear ratio also got deeper, from 2.52 to 2.64.
However, I'm assuming the OP wants to keep his current, perhaps original, transmission.

--------------------------
1979 L82 M21 3.70 FE7
Classic White / Doeskin

 

We used to drive these cars all over the country at 70 mph and turning over 2000 rpm and they worked fine. My cam turns on at about 3k rpm and have a 3200 convertor. With 3:55 gears cruising at 70 and needing to pass somebody the old girl really covers some real estate. A 9 quart oil pan and a good oil pump the extra revs don't bother me a bit.

 

Well said - I agree. Those are basically the standard ratios the factory offered for those engines/transmission combos and in my opinion they got it right given the 1:1 final drives and use cases back then.

 

Correct. My LT-1 did not mind a long 3000 rpm highway cruise at 70 mph one bit. It even got 15 mpg doing it. Now 3500, eh the oil temp started to go up after 10 min or so. Too much past 300* and I started slowing down. LOL

 

It’s more about how the entire drivetrain is set up than just what the rear end gears are.
I’m running 3.08’s in the rear, my 5 speed has a 0.82 overdrive and a 2.87 first gear ratio.
My 496 made 480 ft lbs of torque at 2800 rpm which was where to dyno started recording data and it made close to 600 ft lbs of torque at 4800 rpm.
The engine continued to build hp through 5800 rpm and that’s where we shut it down.
I really wouldn’t want a 3.36 or a 3.55 rear gear.

 

OCB
Now I am not sure....but maybe...just maybe... you are going to have some traction problems!

LOL

 

No problems, just no traction.
I think I’ll just slowly let out the clutch at idle without giving it any gas.

 

If you found a good used 3.55 ratio diff then that is a lot less money than conveting to overdrive. Check the stub axle play in/out the diff. If that 3.55 diff is from a later C3 then it has the weaker slotted clutches but the later, better posi case. May have been worked on previously so no way to know until you look in there.
The 3.55 gears will give you a decent drop in highway rpm over 4.11. Personally I would not even want 3.73 without overdrive, I like to go 65,70 mph and still hear the radio

 

As I am reading here, the 355 makes a lot of difference on highway speeds over the 411. I think my 79 has a 411 ratio, which is barely useable at highway speeds here in Europe (80mph), and I have the standard 3speed w/out overdrive. My question here now is, if the carrier of the 411 would still be fitting for a 355 gear ratio or do I have to get another carrier? New transmissions are very hard to get your hands on in Europe and very expensive, so I will have to find another carrier (way easier to find here) if the 355 gears aren't going on the 411 carrier.

Fyi, I'm not a mechanic and only doing smaller stuff on the car myself, plus english is not my main language, so I might need a dumbed down answer. I do understand basic mechanic terms though

 

In 79 with a 3 speed automatic, the standard rear end is a 3.55. Whether a L48 or L82. With that being said do you know if the previous owner made any kind of change? Cant imagine why he would change to a 4.11 gear with a L48 motor unless there is more to know about your car. The most accurate way to know what rear you have is to spin the wheel one complete turn and count the driveshaft rotation.

With a 255 tire (27.1inch) and 4.11 your tach would be reading 4100rpms at 80mph. With 3.55 it would read @3500rpms.

 

Miles per hour and kilometers per hour makes a difference too.

 

Well I am definitely above 4000 rpms at 80 mp/h (which is 130 km/h, the typical highway speeds in Luxembourg), and I have a stock L48 motor running. I guess the previous owner ( I know nothing about the car before I bought it) wanted it to handle better at lower speeds. It is driving really well at lower speeds up to 55 mp/h (around 90 km/h, so the typical speeds for "land streets", so streets between cities and villages) with enough torque to get going up to those speeds.

 

Check for speedometer accuracy or more likely that your tach is reading correctly.

 

I believe if you put a 3.55 ring on the 4.11 carrier it will have to be shimmed, the 4.11 ring is thicker.

 

Just a note
3:55 is used with a 3 series posi case

411, from GM or from the period (60-80's) uses a 4 series case. The standard 4 series gear is thinner, the old hot rod way when converting to a 4 series gear was to go to your local speed shop and by a spacer. It was a bad idea, but it saved guys money up front over going to the local chevy parts counter to buy the correct 4 series posi case. I will guess that the spacer was under $10 and the new Eaton USA made posi was $200 in 1970's money. That was a lot. Over time the load on the ring gear caused the bolts to loosen and back out, which caused diff failure.

Tom Watt had 4 series gears made by US Gear thicker to eliminate the spacer and not cause a problem. That goes back 25+ years now. Today, Tom's and US Gear are gone. Yukon makes a think 411 vette gear today and charges more for it.

You can still find GM 4 series gears used and they may be in good condition but there were two designs, one was junk and broke, the other is the revised version. Many selling them have no clue what they're selling. Many people rebuilding them today are questionable as well. I have called some out and have answered many phone calls from guys who bought exchange diffs from some places that were bad. Finding a good 4 series posi case is a crap shoot as well because they also had revisions, some were junk, some were good. Most today are worthless because fewer and fewer want 4 series gears and the only new one is from Yukon. All new gears today are imported, mostly from China.

 

Have you actually tried determining what you have by counting driveshaft rotations?
Place the rear of the car on jack stands.
Parking brake off, transmission in neutral.
Mark the driveshaft with paint or chalk or crayon.
Rotate one one of the rear wheels by hand one turn while counting the number of times the driveshaft turns.

 

I have not checked the real ratio, and at the moment I cannot check it because it is in a workshop that changes all the rubber bearings and wheel bearings of the car. The speedometer is extremely off, it shows around 43 mp/h (70km/h) when I do 31 mp/h (50 km/h), the tach is a new one as the old one burnt out last year.

Anyway, the real ratio isn't too important, as it clearly is over 4.00 ratio, and I wanna have a 3.55 as it came from factory. My question was whether I could use the 4-series carrier with the 3-series gears, it seems as if they do not fit. Thank you everybody for helping out!