Guys,
I wanted to get an opinion on my combo.... I have a 72 cutlass with a stock rebuild on a 71 455. It is backed by a TH400 with a B&M shift kit.

The rear in it now is a stock 2.56 one legger.
I have come across a set of 3.73 gears, Ring and pinion only. I would like to know what everyone's thoughts are on this combo. I will have a posi carrier installed as well as new bearings/seals of course.

Just wanted to know what you guys think of the gear selection. It is a street driven/pleasure vehicle. Not really for racing. Just a light to light rice burner....

Would another gear like 3.42 or 3.55 be better for my combo?

Any thoughts/ suggestions would be GREATLY appreciated....

 

You can NOT get a 3:55 ratio for the 8.5" 10-bolt.the 2 most popular gears that I build these rears with,are 3:42 and 3:73. What size rear tires do you have?

 

You are in luck! The difference between 3.73 and a 2.56 is a factor of 1.45, which is almost exactly the difference between 2nd and 3rd gear on your TH400. This means that you can simulate what it will be like cruising around town with a set of 3.73s by instead cruising around town in 2nd gear with your 2.56s. My guess is that, if you do any highway driving at all, you'll feel a little uncomfortable at 70 mph and dial it back to 60 mph. If you're happy driving that slow (or driving faster but spinning your 455 higher), then you'll be OK with the 3.73s.

Of course, this test doesn't let you check out the added acceleration you'll get off the line in first gear with the 3.73s. But you don't really need to perform THAT experiment. We already know you'll like it.

 

I wouldn't go any lower (higher numerically) than 3.42s, you'll be approaching uncomfortable rpm range for extended hiway travel. even with 2.56s that 455 will roast the tires.

beware of the shift kit on wet pavement while making a turn at an intersection. before I removed it I had the rear end break loose and come around at the 1 - 2 shift. several times

 

A good stall converter will mellow-out a shift kit.I've seen plenty of tranny hacks claiming they know how to build a trans,but all they talk about is how hard it will shift.There's a lot more to it than that.Yes,if you have a stock converter,the shifts will be pretty crisp,& possibly a little violent.It sounds like maybe that is what you had Blk71SX. The trans-Go kits are still one of the best.The TCI kits are the same pieces.They just color a few things different to make them look different.Another thing about the trannies is the clutches.You see all these places advertising & bragging up the red alto clutches.Yes,they are good,& better than stock,but di you know that there are 3 other choices beyond those.They also make blue,green,& black.They are colored for the different levels of aggressiveness.I have the blue ones in the car pictured below,but I have other rebuilt TH400's with the green & black.I just haven't had a problem with this trans yet,so I haven't had a chance to try the others.
I'm not sure how I got off-topic with the trans,but it's a good tidbit of info.
If you are running the 14" rims,then you have a tire that is about 26" tall.If this is the case,then I would go with a 3:42 gear.If you have 15" rims,& possibly a 28" tall tire,then you could still consider the 3:73.If you do a fair amount of highway driving,go with the 3:42,regardless of tire size.

 

I thought I wanted to go to 3.42's but my engine builder talked me out of it.. I have 3.08's and thought they would be anemic, but since I am not racing he thought these would be the optimal gears for my 455 and 4 speed, I am running 15" 60 series tires I am hoping this thibng will get up and scoot, but still be "comfortable" at 70+ on the highway... what do you think?

 

3.08 is a great ratio behind a 455 with no overdrive, if you like to drive in the 70 - 80 mph range on the highway. And as long as you have the right torque converter for your engine, I guarantee it won't be anemic.

With 3.23 gears, you'll already find yourself thinking, "You know, this big ol' engine is spinning faster than it really needs to be to push this car through the air." By 3.42 you'll already be on the edge of uncomfortable. With 3.91 you'll find yourself instinctively backing off to 60 mph. Not that you couldn't necessarily cruise at 80 mph for hours on end, but it just feels a little silly.

Another thing to consider when considering low gears like 3.73 or 3.91 is that the car will actually get LESS responsive in many passing situations. If you're in the 50 - 60 mph range and floor it, the transmission may not downshift at all, or, if it does, it will upshift again shortly. So yeah, the rear end is giving you more torque, but the transmission is giving you less. Meanwhile, with 3.08 or 3.23 gears, when you floor it at 50 - 60 mph it WILL downshift and allow you to run in 2nd gear all the way up to the 80 mph range. This not only feels quicker, it IS quicker. There's a reason the factory called 2nd gear "Super."

 

With a bigger (265)/60-15 , a Muncie w/ 1:1 4th, & 3.42 rear, 80mph is 3340rpm...just for ref...

I'll be doing the above, but with a taller tire (275/60 -15) to help drop it to 3280.....plus mine is intended to be an urban sprinter, not a cross-country miler....

All numbers according to
http://www.5speeds.com/rpm.htm

 

Where do you guys live that have speed limits of 80mph.I would like to move there.
Eddie,
Your combination is nothing to complain about.If you have the wide-ratio M20,with the steeper 1st gear,that can compensate for the slightly less rear gear.Even if you don't,you have that 455 to get it moving. The 4-spd cars equipped with AC often had 3:08's or 3:23's. The auto cars equipped with AC could get a 3:42.If you ordered any ratio higher than that they voided some warranties.The higher rpms will also disengage the AC.