Okay, I don't really need down to -40 but need something for real winter weather and figured the lower the temperature rating, the better.

Need something for extra tall and I don't like "mummy" type, I want it to be rectangular.  I also have a backpack type bag but with this one, don't care of it's easy to carry or not, it will be  carried in a vehicle so that easy of carry is not an issue.

Part of the way you get a -40 rating is being a mummy bag.honestly if you want a neg 40 bag that isnt a mummy bag just get a square style as thick as possible and add down or wool blankets until you melt.

If you are willing to go.mummy get any high end down bag that fits you. A snug but not tight fit creates the heat. Yeah they are annoying to sleep in but you get used to it and not freezing is cool.

First post and all that.

But aside from that, for car storage, you can put lighter bags together like a turducken.  And for really cold you're going to want to think about sealing up the neck area so that air doesn't go in and out like an accordion when you move.

USGI 4-piece Modular Sleep System

I have an outfitter style cotton duck rectangular bag that is heavy, huge and comfortable for someone that tosses and turns but even it gets chilly when the temperatures drop into the negatives.  

From my experience with it I will agree with the first reply as I don't think you are going to be able to achieve that sort of temperature rating on a rectangular bag as there is just too much heat being lost in a bag that doesn't conform to your body and have an integral insulated hood.

I used to routinely camp in deep double digits below zero. There is no such thing as a -40 bag. I do not care what the manufacturer says.

What keeps you warm in severe cold?

Elevated off the ground. A good thick sleeping mat is a must.

Light, wicking underwear. You get damp or sweat, you will get cold and your comfort level decreases immediately.

Eat before you go to bed.

A decent pillow.

A wool blanket over the sleeping bag.

That will keep you warm close to -60. Which is close to the coldest I ever made it overnight camping.

Wiggy's is your answer. Check them out. Good stuff.

I have a feathered friends bag that goes to -40.    Not sure if they are still around though.  $700 20 yrs ago

Wiggys makes a hooded rectangular bag. I have two of them, they’re heavy as fuck and the shell fabric has zero wind resistance, so plan accordingly.

I used a feathered friends bag on Denali. Those things are miraculously warm.

My personal cold bag is a rei expedition bag with a -20 rating. I have been comfortable at -20 with the addition of a hot water bottle.

My general use bag is a kifaru 0 degree slick bag, but I don’t think I’d want it lower than about 10*

WOW They have really gone up in price. 15 or so years ago a guys was selling them for $50ea shipped on the EE and bought 4 of them.

Mark Taylor (Veteran owned) has made some great outdoor gear over the years, I don't know if he still owns it.

Wiggys is making very little now.

Missed my chance for some nice winter gear.

Car camping?  Easy-peasy.  Get a big, heavy, inexpensive sleeping bag.  I got my kids some 0-degree rated Coleman mummy bags (sorry, but sleeping bags for winter weather are all mummy bags, and for good reason).  With an air mattress under them and wearing some heavy pajamas, even when the temp has been below freezing, they always have to open the bag up a little to keep from being TOO warm.

Or, get yourself a cot, and a foam pad for the cot.  In addition to being super comfy, having a couple inches of foam underneath you gives you an extra bit of insulation that really helps.

Or... get a 200-400WH powerbank/inverter, and a heating pad, keep the heating pad running in your sleeping bag with you all night long.

I have a very expensive Big Agnes bag and a very expensive lightweight insulated sleeping pad for when size and weight are a concern... but I'd never buy that combo for car camping.  

Finally...wear a purpose-made base layer under whatever thermals you wear at night.  They do definitely make a difference.

Cot and foam pad on top….the thicker the better. Don’t sleep on an air mattress in cold weather….I made that mistake once….never again.

A good sleeping bag of whatever style you like in as low of a temp rating as you can get. Cot with 3 inch or better foam pad. Then, buy a regular rectangular sleeping bag, unzip it, and toss it over the top of you.

Wear a watch cap to bed. You need to breathe but also need to retain the heat your body makes.

A hot water bottle at the foot of the sleeping bag is nice to keep your feet warmer for awhile.

Eat something right before you go to bed. Preferably something like bread because it makes heat as it digests. Keep some more close by and halfway through the night, eat some more.

I've done quite a bit of winter camping in northern MN and Canada. Already mentioned but very important is a closed cell foam ground pad. I've used Thermarest but I've also spent some cold sleepless nights when they went flat.

I also use a vapor barrier liner inside the bag. It adds a few degrees of warmth but more importantly keeps your perspiration out of your bags insulation. When you are out for an extended period of time its difficult to dry out your bag. When Will Steger went to the North Pole they slept in their clothes in case the ice cracked. But their sleeping bags turned into blocks of ice and they ended up discarding a few of them. Then they slept together in the remaining bags.

Layer a couple of lighter bags.
Like a 20 degree in a 0 degree.

https://support.enlightenedequipment.com/hc/en-us/articles/115002770588-How-to-layer-quilts-for-sub-zero-camping

I have a Montbell Spiral Down -20 bag. It allows for stretching and movement while still being a mummy bag.

Then I add a Katabatic Gear 22 degree quilt for temps under -10.
Plus use a Thermarest Xtherm and a closed cell foam pad for ground insulation.

wiggy's.

Hope this is not off topic, what did our GIs use in the Korean War??
I have a large chicken feather bag from the 60s? I think it weights about 10 pounds. It works, but have not tried it in minors temps.

ive used both western mountaineering and feathered friends in real cold, around -30. you must get a great pad, or better yet 2 or more great pads, for anything to be effective at that temperature.

wiggys has a good rep for synthetics

Yup, Wiggy's.

Other than that, you can look at Cabela's, Sportsman's Guide, etc for their stuff.  It's probably going to be a multi-bag system.  A GOOD pad is required. Air isn't the best insulation...so foam is better.  Get something with a good closure around the head/neck and/or bring an extra blanket to use to seal any gaps.  Get a good beanie hat, as well.  I like ones that cover my nose when pulled down.  Wear some light to mid weight long johns (goal is some retention but to let some heat into the bag to keep extremities warm.  (Wore a parka and super heavy pants once and my feet were COLD all night.)

I already have a Wiggy's bag, had it for 20 years but not entirely happy with it, not sure the temp rating, it seems to be good down to about 0, I think and the mummy type is just uncomfortable.

I like this one the most out of their selection:

https://www.wiggys.com/sleeping-bags/hunter-antarctic-rectangular-sleeping-bag/

Edit: Can someone explain the zipper configuration?

Standard Zipper: A full 118" zipper that extends down the side and across the bottom of the bag (comes standard on all Wiggy's rectangular sleeping bags). This allows the bag to be fully unzipped on the side & bottom so that you can spread the bag out like a comforter. Two bags with 118" zippers can be 'mated' by unzipping both bags completely and placing one bag on the bottom (lining facing up) and one bag on the top (lining facing down). Then zip both bags together.

*Note: Mating two Hunter model sleeping bags with hoods together will result with one person having the hood under their head and the second having the hood in their face.

65" Side Zipper: Available on either the right or left side of the bag. This allows one right-zip & one left-zip bag to be mated together in traditional fashion, like our mummy-style bags.

These are the shizz. I used this as my model. Two mummy bags, 36* left zipper, 20* right zipper, on top of a foam bed roll in hammock, add under quilt when really cold.  I have been toasty unto single digits. I have no idea how cold it could get before I notice. But I had to unzip at 26* one morning.

Also, I sleep in nothing but undies for best thermal activity. This helps with the claustrophobic effect.

ETA: I’ve awakened to ice on my mosquito net many mornings.

For the non poors.

What was the issue? I've only been down to low single digits but I had no issues on an insulated therm a rest. I use a stuff sack to inflate so I don't lose loft when it cools down.

OP- there are very few mummy bags that will do -40, I doubt anyone makes a rectangle bag that actually goes that cold, it would be extremely hard to engineer. You need like 10" of loft even with a really well engineered mummy bag. Definitely echo the ground insulation comments, and probably layering more than one pad.

If I had to try to accomplish what you wanted, I'd get some really warm down booties, down pants, and down jacket and wear them in the warmest rectangle bag you can find.

If you are planning on the contingency of maybe someday having to sleep in -40 weather, that's a life and death situation. Buy the best bag you can afford and you will have to have an insulated sleeping pad-the best sleeping bag in the world is cold when it's insulation gets compressed under your weight and you are laying on the cold ground.... I know this because I got hypothermia, lost my ability to regulate temperature for a few hours even with a hot shower and bed after not making it thru a night testing a sleeping bag in 20 degree weather on the ground-it was scary.

I'd be looking at a Feathered Friends down bag and the best sleeping pad you can find. I've had a lot of sleeping bags over the years, my FF bag is the only one that I would agree that you are warm at it's rated temperature.

Still are, I visited their shop and got mine in person-one was a floor model or return (can't remember) on sale and the other one had to be made. I just checked pricing on their website and it was about $700 for a 10 degree winter bag, think I paid $600 for mine in 2016, and it's a 0 degree bag. Nice people who are clearly the granola cruncher type, but didn't have a problem selling OD green bags to someone clearly not like them. Would love to give them more business in the future.

This will get you to -30
Browning Klondike Sleeping Bag
https://www.midwayusa.com/product/1018488333?pid=472569

They had clearance sale a few months ago on browning bags.

I don't have this bag, but as other said add a wool blanket or two.

Oh yes, minus 40 degrees! The trivial pursuit temperature which is the same in Fahrenheit or Celsius. Funny that's what the OP requested.

Air mattresses get COLD. Thermarest mattresses actually insulate you from the ground similar to a piece of foam. I’d much rather sleep on a thermarest mattress (and have) than to sleep on an air mattress in cold weather. I now have 3 inch foam pads on top of a Coleman cot and with the appropriate sleeping bags and/or blankets, I’m warm as can be.

It's been a long time since I needed anything below 10 degrees. While I don't care for the MTSS or Wiggys for backpacking, truck and cabin storage-use, these are hard to beat. Wiggs FRTSS is better and gives you a wider range of temps, and if you want a rectangular bag:

Hunter Ultima Thule - Rectangular (minus-20 degrees)

Hunter FTRSS Overbag (40 / -60)

The FTRSS overbag is rated for 40 degrees by itself and when paired with the Thule above, both are rated together down to minus-60 degrees.

I really don't care for Wiggys if stuffing in a pack, but their shit is rock solid. I have their 0-degree Super Light bag and did that test of soaking my clothes and climbing into the bag with overnight temps around 20 degrees, and I shit you not, my clothes were dry by morning. I was shivering for a while, but warmed up quickly and then nodded off...it helps when you're tired as shit too.

ROCK6

I like Butler bag.

My best (semi-informed and non-Expert) guess is that very High-End synthetic sleeping bag fill is slowly crawling up to the level of High-End down fill.

Reckon that getting "High-End" insulation will not be cheap, regardless of type.

Getting true, High-End down is not easy, as there seems to be no "Universal" standard for such item.  In short, "Buyer Beware".  Evals of syn insulation are also questionable.

My 1970 Eddie Bauer KaraKoram parka was advertised by respected mfr as having "Prime Canadian Goose Down".  Still own it, and it still works fine in cold/dry arctic temps. Requires a breathable wind and rain barrier for use in differing, non Arctic, non-windy climates.

Current go-to in extremely low temps is LL Bean Baxter State Parka.  Is much heavier, what with more durable Gore-Tex outer fabric.  OTOH, mostly water-resistant as long as DWR is regularly applied, and completely windproof.   Is stuffed with down, apparently of decent quality.

IDK how old it is, bought it at a thrift store for $30.  Caling it a "steal" is a vast understatement.

Taking both the EB coat and the LLB coat in comparison, a careful user could slip a waterproof and windproof outer cover over the EB KaraKoram coat and have lighter weight and better "packability" in addition to wind/water resistance.  Maybe similar level of insulation, but have not had sufficiently low (sub-zero) temps to make a side-by-side comparison.

Needless to say, neither coat is currently offered in the same configuration as originally offered, and I suspect the quality of down used in both coats has not at all improved.

Unless living in purely Arctic (Cold/Dry) conditions, I always suggest syn fibers for both garments and sleeping bags.  Yes, high-quality down (assuming you can find it!) is better insulation than most syn fibers, but down is a pure bitch to dry out once it gets wet.

Trying to dry down sleping bags and garments requires extreme measures.  I recall a photo in one of Shelby Stanton's uniform/equipment books (Korean War) showing two GIs using a fan-driven heater and 6" flx hoses drying-out USGI down sleeping bags.  

BTW, most USGI "down-insulated" Sleeping bags are junk.  the tag inside the bag states "crushed waterfowl feathers".

Side-Trip: Even the "Arctic" USGI bags are heavy junk compared to what is available, but FAR better than typical USGI sleeping bags.  Make certain you have the cotton canvas cover for all versions.  Cotton covers are, at best, water-resistant, but chems can be applied to them to make the covers decently water-repellant.

I'll wear my "legacy" waterproof(ed) down garments near the house, but if in the field, I'd suggest syn fibers for insulation and relatively easy shedding of water.

Enough of my blather.

Best wishes to you all!