# New to Forum, 67 GTO 3.73 posi overkill?



## GreenMachine (Jul 1, 2010)

Hey guys I'm new to the forum! 

I'm planning on redoing my rear diff next week and need some advice. 

Stats on car:
67 GTO 400cui bore over .30, forged steel pistons, mild cam, headers, street avenger 670 carb, edel performer intake mani, TH 400 trans with 2,400rpm stall and mild shift kit.
Car makes about 350whp/400wtrq...

So my question is do you think 3.73 gearing is a bit overkill? I try to avoid highway driving as much as possible (rock chips, etc) but I do drive a couple 30-40mile trips to car shows and what not. How bad is 3.73 on the highway compared to say 3.55? 

Any advice would be greatly appreciated!
Here's a pic of my car...


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## GreenMachine (Jul 1, 2010)

Hey guys just did some late night reading... I'm going to figure out what gearing the car has in it right now (ill check it this weekend) and probably go from there. From the posts I've been reading it seems like a 3.55 is a good happy medium... i feel like 3.31 wouldn't be as aggressive as I'd like.


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## ALKYGTO (Mar 29, 2010)

Pretty car, and nice scenery too , are you in Florida? If you have the 3:55's, I would stay with those. Nice all around gear, plenty snappy without overrevving on the highway at 65.


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## Eric Animal (Oct 28, 2007)

Unless otherwise ordered, all 1967 GTOs with factory A/C and the t400 tranny were built with a 2.91 rear....3.23. and 3.36 optional NOT recommended to go over 3.36 due to higher cruise rpms and engine temp/ AC issues. If you have an interest, check out a 4 speed automatic conversion......a tranny with a .70 OD would make a 3.73 rear 2.61 for highway crusin'. Ericarty:


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## geeteeohguy (Feb 2, 2008)

Check the "gear ratio" sticky thread at the top of the post area. Good info there. I guess you already have the 4-series carrier for your 3.55 gears? A stock, 2.93 ratio which you probably have (A/C auto's in '67 had this ratio most often) is a 3-series carrier, and will accomodate up to a 3.23 gear. If you want to run 3.36 and up, you need a different carrier. My recommendation would be keep the gears you have, or go to 3.23's. I pulled the 3.36 rear out of my '67 after 27 years of driving, and swapped in a 2:56 posi. I love it, but I take my car on long trips cross country, so fuel mileage, comfort, engine life, and speed are important to me. Your car is drop-dead gorgeous, BTW.


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## Rukee (Feb 8, 2007)

:agree
A good way to look at the gear ratio is it's the # of times the drive shaft turns to make the tires turn over 1 full revolution. With a turbo 400 a 2.9x to 3.2x would be great.
Tires size also contributes to engine RPM on the highway. The shorter the tire the more RPMs it's going to turn at speed, taller tires drop the RPMs.


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## GreenMachine (Jul 1, 2010)

ALKYGTO said:


> Pretty car, and nice scenery too , are you in Florida? If you have the 3:55's, I would stay with those. Nice all around gear, plenty snappy without overrevving on the highway at 65.


Hey Alky, thanks! Nope not florida, I'm in So Cal. I'm not sure what the car has in it now, I have to check the stamp on the rear end. All I know is that it's stock forsure b/c the two previous owners never touched it.


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## GreenMachine (Jul 1, 2010)

geeteeohguy said:


> Check the "gear ratio" sticky thread at the top of the post area. Good info there. I guess you already have the 4-series carrier for your 3.55 gears? A stock, 2.93 ratio which you probably have (A/C auto's in '67 had this ratio most often) is a 3-series carrier, and will accomodate up to a 3.23 gear. If you want to run 3.36 and up, you need a different carrier. My recommendation would be keep the gears you have, or go to 3.23's. I pulled the 3.36 rear out of my '67 after 27 years of driving, and swapped in a 2:56 posi. I love it, but I take my car on long trips cross country, so fuel mileage, comfort, engine life, and speed are important to me. Your car is drop-dead gorgeous, BTW.


Thanks for the kind words about the car! That's the exact thread I found last night after posting. Lot's of good info... I'm pretty sure the car has 2.93 in it now but I will recheck it this weekend. 

I feel like the rear end is the weak point of the car right now. If I'm doing some spirited driving from light to light I feel like the car has too much torque. First gear is too short and I feel like the monster chirp shifting into 2nd gear is going to be the last thing I hear one of these days before the rear diff breaks. I'm going to add a posi in it so I thought might as well play with the gearing a bit and make sure I don't have any weak links.

I'm 26 years old, and I've had this car since I was 15. Everyone I used to run into had 3.73 or lower (4.10,etc). I'm now worried about engine wear cruising on the freeway but I definitely want something that's going to compliment the bolt on mods and power that the car makes down low. I feel like the posi and gearing is what's holding the car back for some reason. Hard to explain but hopefully you get what I mean...


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## GreenMachine (Jul 1, 2010)

Rukee said:


> :agree
> A good way to look at the gear ratio is it's the # of times the drive shaft turns to make the tires turn over 1 full revolution. With a turbo 400 a 2.9x to 3.2x would be great.
> Tires size also contributes to engine RPM on the highway. The shorter the tire the more RPMs it's going to turn at speed, taller tires drop the RPMs.


Thanks Rukee, yup i was planning on getting the number off the rear end and also doing the test you suggested to make sure both match. I'm 99% sure it's stock. I run cragar SS 235/60/14 in the front and 235/60/15 in the rear. I'm planning on hopefully widening the rear to atleast a 255 but that won't effect the tallness of the tire, probably still stick with a 60. I like the meaty/square looking sidewall look. I have to check the width of the rear wheel, I may have to order a new set of Cragars if I can't fit at least 255s on it but I don't know how wide I can go in the rear without cutting anything or doing any mods. Do you happen to know how wide I could go if I order a new set of cragers with specific backspacing?


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## Rukee (Feb 8, 2007)

GreenMachine said:


> Thanks for the kind words about the car! That's the exact thread I found last night after posting. Lot's of good info... I'm pretty sure the car has 2.93 in it now but I will recheck it this weekend.
> 
> *I feel like the rear end is the weak point of the car right now. If I'm doing some spirited driving from light to light I feel like the car has too much torque. First gear is too short and I feel like the monster chirp shifting into 2nd gear is going to be the last thing I hear one of these days before the rear diff breaks. I'm going to add a posi in it so I thought might as well play with the gearing a bit and make sure I don't have any weak links.*
> 
> I'm 26 years old, and I've had this car since I was 15. Everyone I used to run into had 3.73 or lower (4.10,etc). I'm now worried about engine wear cruising on the freeway but I definitely want something that's going to compliment the bolt on mods and power that the car makes down low. I feel like the posi and gearing is what's holding the car back for some reason. Hard to explain but hopefully you get what I mean...


It should have a posi already. A lower gear ratio will make that 1st gear is too short and make it even shorter feeling. My car had 3.90s when I got it and I changed it back to the original 3.55s. It felt like it grow legs between the gears, each gear pulled harder longer.
...and don't worry about breaking the rear end, very few have broke in 40+ years. Just run what ya brung.


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## Rukee (Feb 8, 2007)

GreenMachine said:


> Thanks Rukee, yup i was planning on getting the number off the rear end and also doing the test you suggested to make sure both match. I'm 99% sure it's stock. I run cragar SS 235/60/14 in the front and 235/60/15 in the rear. I'm planning on hopefully widening the rear to atleast a 255 but that won't effect the tallness of the tire, probably still stick with a 60. I like the meaty/square looking sidewall look. I have to check the width of the rear wheel, I may have to order a new set of Cragars if I can't fit at least 255s on it but I don't know how wide I can go in the rear without cutting anything or doing any mods. Do you happen to know how wide I could go if I order a new set of cragers with specific backspacing?


I like the big tire look too. I'm running 295/50/15s in the back with a 4 3/4 backspacing. Had to trim just a bit from the inside fenderwell with a cut off tool, but they look badass back there. :cheers


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## GreenMachine (Jul 1, 2010)

Rukee said:


> It should have a posi already. A lower gear ratio will make that 1st gear is too short and make it even shorter feeling. My car had 3.90s when I got it and I changed it back to the original 3.55s. It felt like it grow legs between the gears, each gear pulled harder longer.
> ...and don't worry about breaking the rear end, very few have broke in 40+ years. Just run what ya brung.


So if my car has 2.91 I'm going to lose even more 1st year if I go 3.55 huh? I think I've already taken 3.73 out of the picture, so now I'm debating on 3.55 or 3.31...

The guys that dyno'd my car said that everything is done perfectly and that the only thing the car needs is posi and gearing to really come alive.

As for the posi, WEIRD story Rukee... Back in the day when I used to race my buddies in highschool it was always a peg leg, but then for every 10 burn outs one of them would some how engage the other wheel and I'd leave two lanes of rubber. Is the posi broken or something? Honestly Rear diff/gearing/posi is all stuff that I have poor knowledge on. Everything else I have no problem with and have the balls to tinker with myself, but the rear diff is overwhelming thats why I'm having a reputable local shop do it for me.


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## GreenMachine (Jul 1, 2010)

Rukee said:


> I like the big tire look too. I'm running 295/50/15s in the back with a 4 3/4 backspacing. Had to trim just a bit from the inside fenderwell with a cut off tool, but they look badass back there. :cheers


I love your car, I was up late last night looking at your videos and photo album. Love the fatty tire look in the back!


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## ALKYGTO (Mar 29, 2010)

What MPH does your car shift into 2nd gear? Yeah, if it's got 2:93's a 3:55 will shift into 2nd gear "quicker" because you'll reach the shift point sooner. 

Open diff's send the power to the wheel with the least resistance, so if everything was perfectly balanced (ie. buddy in the pass seat) you _could _spin both tires.


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## GreenMachine (Jul 1, 2010)

ALKYGTO said:


> What MPH does your car shift into 2nd gear? Yeah, if it's got 2:93's a 3:55 will shift into 2nd gear "quicker" because you'll reach the shift point sooner.
> 
> Open diff's send the power to the wheel with the least resistance, so if everything was perfectly balanced (ie. buddy in the pass seat) you _could _spin both tires.


I'd have to check what MPH it shifts into 2nd, but doesn't it very on how long you keep it in or how much your on the pedal? I feel like first gear shifts into 2nd at like 10 mph when I'm cruising but if I'm on it shifts later...

Yeah I'm pretty sure it's an open diff...


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## geeteeohguy (Feb 2, 2008)

It would be an open diff unless a posi was ordered. Stock gears of '67 was 3.36 for non AC cars and 2.93 for AC cars. With the 2.56 gears I put in mine, 1st gear is good for 65 mph, and second for 105mph. If you are feeling "short legged" with the present gears, yu need to check the ratio. If it's a 2.93 or a 3.08 or a 3.23, leave the gears alone. Just add the posi unit and run it. You will be happy, trust me. If you put in steeper gears, you will be even shorter legged, have less top speed, more revs per mile, more engine wear, hotter running engine, etc. etc. Pontiacs are NOT small block Chevrolets. They do not need a 4:11 gear to be quick. They have a ton of torque, and can easily pull a longer legged, more user friendly gear. With gas at $3+ per gallon, lower revs on the road are a good thing!


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## GreenMachine (Jul 1, 2010)

geeteeohguy said:


> It would be an open diff unless a posi was ordered. Stock gears of '67 was 3.36 for non AC cars and 2.93 for AC cars. With the 2.56 gears I put in mine, 1st gear is good for 65 mph, and second for 105mph. If you are feeling "short legged" with the present gears, yu need to check the ratio. If it's a 2.93 or a 3.08 or a 3.23, leave the gears alone. Just add the posi unit and run it. You will be happy, trust me. If you put in steeper gears, you will be even shorter legged, have less top speed, more revs per mile, more engine wear, hotter running engine, etc. etc. Pontiacs are NOT small block Chevrolets. They do not need a 4:11 gear to be quick. They have a ton of torque, and can easily pull a longer legged, more user friendly gear. With gas at $3+ per gallon, lower revs on the road are a good thing!


You make a very good point GTOguy. Are you a 4speed? B/c if not and your 1st gear goes to about 65mph then I'm guessing maybe someone did put some gears back there at one time or another. I'll check it ASAP, now I'm super curious. 

PS Is there some type of calculator online where you can put in your trans type/gear ratio and it will feed you the gear shifts and what not?


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## geeteeohguy (Feb 2, 2008)

Yes, my '67 is a TH400 automatic. If it were a 4 speed, I would not even consider a 2.56 gear. 3.08-3.23 would be more appropriate for a stick car. I have 3.36 gears in my 4 speed '65, and they were a huge improvement over the 3:55 and 3:90 gears I previously ran in that car. You can go to Wallace Racing online or many other websites, including 4x4 websites, and get free gear ratio calculators. They are a great way to determine road speed vs engine rpm. I've driven these cars for over 30 years, with all kinds of gears, and believe me, if you drive on the street and rarely see the strip, longer legged gears are much more user-friendly. Yes, 3:90's are quick, but having the engine pulling 3500rpm at 65 mph or so, and having a top speed of a little over 100 is terrible in everyday driving. My '67 cruises 75mph at 2450 rpm, and delivers over 19mpg at that speed. At 3000 rpm, the road speed is 92 mph. I find myself actually driving the car more, now that it gets the same mileage as my 4Runner!!!


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## Rukee (Feb 8, 2007)

I should swap for the 3.36s. I'm only getting 11mpg with the 3.55s, and I love to cruze the car all around. Better gas mileage would be great.


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## geeteeohguy (Feb 2, 2008)

Rukee, I put in the 3:36 gears during an overhaul of the rear back in 1990. I have the same 4-series carrier you do, and I had run 3.55 gears, etc. Yes, they are a real blast around town, but I wanted a bit more cruise-a-bility. The 3.36's still pull very well out of the hole (still no traction----you know how it is!!), but they feel a lot better on the highway. I'm at 3000 rpm at 70mph with a 26 inch tire, and with the 27 inch tire I'm currently running, it's more like 3000= 73mph. My revs were lowered just enough to make 70mph a pretty sweet cruising spot, where with the 3.55's, it was more like 60mph. The 3.90's? even slower! For years, you could only get used 3.36 gears if you were lucky enough to find them. Now, I see that AMES has them, along with their 3.55 and 4.11 sets. Changing my gearing to 3.36 really changed the way the car feels: less "buzzy', less noise, better power at speed. One of the best things I did to the car (besides having my driveshaft trued and balanced!!) I say go for it.


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## Rukee (Feb 8, 2007)

Crap, I just bought an Aburn Carrier last year or so. :/


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## geeteeohguy (Feb 2, 2008)

Ruk, if the Auburn carrier works with your 3:55 gears, it should work with the 3:36 gears. The same, 4-series GM carrier will work with 3.36, 3.55, 3.90, and 4.33 gear sets. Not familiar with the Auburn, but the dimensions/compatability should be similar! Should be a direct, bolt-in swap. Not cheap, tho'. Ames wants $319 for their gear sets.


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## 66tempestGT (Nov 28, 2009)

the difference between 3.36 and 3.55 at 75 mph is about 150 rpms.


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## Rukee (Feb 8, 2007)

Going from 3.90s to 3.55s doesn't sound like that much difference either, but it was night and day. Made the difference of being able to drive the car out of town versus it staying in town. 3.90s it would just scream on the highway.


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## jetstang (Nov 5, 2008)

Rukee said:


> Going from 3.90s to 3.55s doesn't sound like that much difference either, but it was night and day. Made the difference of being able to drive the car out of town versus it staying in town. 3.90s it would just scream on the highway.


You could get 275 60s and pick up some tire diameter over your 295 50s. Get an Edelbrock carb and metering rod kit and just lean it out, that will help mileage alot. My stock 400 with a 750 Holley got 10 MPG and wasn't fast. My 350 has an edelbrock on it and I hope the MPG is way better.


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## geeteeohguy (Feb 2, 2008)

3.55's to 3.36 isn't much. But like Ruk said about the 3.90's, there was enough of a difference in my car that I really appreciated the 3.36's. Changed the car. I take mine on the freeway every time I drive it, and I like to drive it. If I were just going local, and going to shows, etc., that would be different. If I had just spent a lot of effort and $$$ on 3.55's and an Auburn unit, I would probably "live with it" unless I had a bee in my bonnet to change something. A 27 or 28 (if it'll fit) inch tire would help, too.


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## Rukee (Feb 8, 2007)

jetstang said:


> You could get 275 60s and pick up some tire diameter over your 295 50s. *Get an Edelbrock carb and metering rod kit and just lean it out, that will help mileage alot. My stock 400 with a 750 Holley got 10 MPG and wasn't fast. My 350 has an edelbrock on it and I hope the MPG is way better.*


...and ditch the factory correct Tri-Power set up??? Are you serious?? Really, where is that 'One Touch Ban and Clean' button again?!?!?:willy::willy:


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## Too Many Projects (Nov 15, 2008)

I have an original posi with 3.55 gears. I don't think I will need them since I put a 12 bolt under the car. I really don't know what it's worth, but I may be persuaded to sell it. Anyone have an estimate of value ?

Nice car too..........:cheers


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## GreenMachine (Jul 1, 2010)

Hey Guys checked the gear ratio today, 2.73 is what we came up with. I think I'm going to go 3.36 or 3.55 when I put in the posi. But I'm probably leaning towards 3.55, even though I have a feeling I should go 3.36. lol I don't know! Such a hard decision!

Everyone at the shop and elsewhere say go atleast 3.55 and everyone on here is pretty much leaning towards 3.36!


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## 66tempestGT (Nov 28, 2009)

if thats the toughest decision you have to make this week, life is good. :cheers


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## 68greengoat (Sep 15, 2005)

Too Many Projects said:


> I have an original posi with 3.55 gears. I don't think I will need them since I put a 12 bolt under the car. I really don't know what it's worth, but I may be persuaded to sell it. Anyone have an estimate of value ?
> 
> Nice car too..........:cheers


Throw me a price I can't refuse! I want to put 3.55's in mine. I too only have 2.73's. But, I rarely get above 60mph since I'm mainly on 2 laners in the bush dodging deer. As you know, delivery arrangements would be easy....
PM sent....



GreenMachine said:


> Hey Guys checked the gear ratio today, 2.73 is what we came up with. I think I'm going to go 3.36 or 3.55 when I put in the posi. But I'm probably leaning towards 3.55, even though I have a feeling I should go 3.36. lol I don't know! Such a hard decision!
> 
> Everyone at the shop and elsewhere say go atleast 3.55 and everyone on here is pretty much leaning towards 3.36!


Sounds like if you do alot of freeway driving go with the 3.36. If not, like me, go with the 3.55.........


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## GreenMachine (Jul 1, 2010)

Hey guys, I slept on it...I think maybe I should just go with some 3.23s or 3.36, 3.55 would be too much I think. And with a future goal of putting A/C back in the car 3.55 would probably be a bad idea. After doing a WOT run yesterday, I feel like the car gets out of the hole quick enough and 3.23 would give it a big enough boost down low but keep cruise-abilty up top. That way engine wear wouldn't be as much of an issue. 

Does anyone have a Th400 with A/C and 3.23 or 3.36? What do you guys think or it? Any guesses as to what rpm I'd be at when cruising at 60-65? (2400rpm?) Also I've never driven my car with the A/C on(didn't work when I bought the car) so I've never experience the effect that it has on the motor (temps/rpm/performance). Sorry to beat a dead horse guys, but rear diffs were always my point of weakness when it came to car knowledge and I'm getting a crash course on this forum and learning tons. 

I'm leaning towards 3.23 b/c that was an upgrade option for Auto A/C cars BUT I'm entertaining 3.36 b/c that was the standard for Auto non-A/C cars and I don't know the effect the A/C has on the motor... Ugh I wish they were easier to swap out and less expensive, then we could experiment a bit!


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## Eric Animal (Oct 28, 2007)

2.93 to 3.23 will definately be noticable for example if you had a 2.93 rear and were cruising at 2400 RPM then switched to a 3.23 you would be turning 2645 RPM at the same speed...basically.


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## jetstang (Nov 5, 2008)

Just get 4.56 gears and a gear vendor overdrive!! My 454 SS has 3.73s and a gear vendor and I cruise 3000 rpms at 70, but still gets 10 MPG lol..
In my Lemans, I replaced a 2.76 stock rear with a 10 bolt chevy with 3.36 gears with a turbo 350 trans. It really woke the car up down bottom. Now, the speedo is about 67 at 55 mph, I haven't changed the trans gear. Either gear is a nice compromise gear, 2.76 stock to 3.73s/4.11s. The car should act better with the ac on, as it's easier for the car to get going and with more RPM the AC is spinning quicker, soo... The car may run hotter, but the fan is spinning faster.


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## geeteeohguy (Feb 2, 2008)

Green Machine, like all of us Pontiac guys who DRIVE GTO's every day are telling you, there is a noticable difference between gear ratios, even if the rpm charts state small rpm changes gear to gear. The car will feel different. You originally stated that the car felt too short legged around town and that it shifted too quickly through the gears, etc. If you change your current 2.73 ratio to a 3.23 or a 3.36, your problem will get WORSE, not better. The guys telling you to run 3.55 gears don't own your car and won't be driving your car. You own it and you drive it. My honest recommendation: leave the 2.73 gears in it and add a limited slip to it, or, number two suggestion: change out to a 2.93 limited slip diff. A 2.93 in an auto ac car is a great ratio, great off the line, great on the highway, great top end speed, with unreal passing power at 60-120 mph. Believe me. If you are going to drive this car on the street, use the AC, and are looing for a fast, comfortable, semi-economical cruiser, leave the gears alone. It's your car, so do what you feel is best. This is just my opinion, based on what has worked for me owning and driving these cars since the 1970's.....


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