How do I determine diff. year and is 3.91 a good gear ratio?

[WH23G3G]

I was able to remove the center chunk without any problems, it took like 15 mins. I unbolted it and let it drop onto the plastic oil drain tank that caught the old fluid and it was fine. I cleaned it off and found out it was a 2070741 with a 3.91 gear ratio and the Sure Grip. How do I find out what year this one is from? I did a little researching online and found the 741 was available 1964-1972. The same site also went into detail on the different versions of the 8.75. It said the 741 was the weakest of the castings used in low horsepower applications and had the small 1 7/8" stem pinion. There were no problems with the differential or axles when I got the car so I was just going to put new seals and fluid in. Is 3.91 a good gear ratio to have for a 73 with the stock 400-4bbl and 727? Any recommendations on fluid to put back in? Synthetic or conventional? 80w-90 or 75w-140? If I replace both axle seals, pinion seal, and carrier gasket is synthetic ok to use?

[2Gunz]

I just switched mine to 3.55's

IMHO 3.91 is way to high if you are going to do highway cruising.

The 3.55 I just put in are borderline.

On a side note I have a 3.55 gear set for a 741, thats brand new in the box.

Ill let it go for a good price.

[Ghoste]

But OTOH, if you don't plan to go screaming down the highway for hours at 70mph, the 3:91 are going to get that big ol' boat up to the fun zone of the rpm range a lot quicker.
In other words, you have to decide on your primary use of the car first.  As for a stock 400 and an automatic, the 741 casting will hold up a lot better than you think.  It may be the weakest casting but it's one of those sliding scales.  The weakest 8.75 is better than some of the other makes strongest IMO.

[1969chargerrtse]

I pulled out my 4:11's and dropped in a 3:23.  Maybe a little low but it was a rear that also came on some 4spds and I "may" put one it.  Otherwise it should be nice to me MPG wise over higer gears.

[John_Kunkel]

A manufacture date is stamped on the ring gear, this gives a good indication of the year of the pumpkin.

[r73mopar]

i run a 3.23 in my 73 440 & 727 witch runs pretty good but like others said it depends on what you want to use the car for. i would actually prefer 3.91 in mine but mine just cruises around town 

[mikesbbody]

as r73mopar said, it depends what you use the car for. IMO 3.55's are a good gear and are not too much on the highway. Dont sweat it about the 741 they used those in the super stock cars so they cant be that bad!

[doctorpimp]

Behind a stock 400 4bbl, a 741 is strong enough IMHO.  The 400 2bbl came stock with a 8 1/4 and they held up OK...
The 3.91 will give you the acceleration that you'd like with a musclecar. 
Run that for now and IF you personally don't like it due to gas$$$  then look for a lower # gear 3.23 or 3.55.

[Bandit72]

I have 3.91's for my charger, and another center "chunk" with 3.23's i can swap in if i ever go on a long haul.

[bill440rt]

Behind a stock 400 4bbl, a 741 is strong enough IMHO.  The 400 2bbl came stock with a 8 1/4 and they held up OK...
The 3.91 will give you the acceleration that you'd like with a musclecar. 
Run that for now and IF you personally don't like it due to gas$$$  then look for a lower # gear 3.23 or 3.55.

That 741 will be plenty strong in your stock application.
I also agree that, for most situations, a 3:55 is probably the best "all-around" gear for street cars. You probably don't wanna go any higher for the highway, maybe a 3:73 but that's it.
A 3:55 gearset puts you at about 3000-3500 rpm's in the highway, cruising at 60-70 mph. My '69 has 3:91's, but I plan on installing an overdrive trans.

[RECHRGD]

Yes, 3.91's are too steep if you do much highway driving.  I went to 3.55's and they were OK, but I still was winding too high for my taste in high speed traffic (75/80).  I now have a gear vendors O/D with the 3.55's.  Nothing passes me now!    Bob

[Rolling_Thunder]

I recently built a stout 440 for a 69 roadrunner...     the test was driving it to Mojave and back with its 3.91 gears at 70mph - It was a little un-nerving though...     and loud