# Rear end question



## xconcepts (Jan 4, 2009)

I asked this on the gear ratio thread but didn't get an answer. 

I have the original rear end for the 66 sitting in storage but is in good condition, sitting at 3.55 ratio. Currently in is another rear end, don't know the model or year, but is a 2.56 ratio. I want to go to a 3.23, so would i be better off changing the original 66 rear end or the one currently installed?


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## 67/04gto (Jan 15, 2005)

i would say use the 3.55 rear casue you only have to change the gerars.if you use the 2.56 u would have to use a 3 seriers carrier and gears.


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## BMR Sales (Feb 12, 2009)

67/04gto has the right idea. Thats going to be your best bet.


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## geeteeohguy (Feb 2, 2008)

You can't use either rear end. 3.23's will not work with a 4 series carrier (3.36-4.33). You need a 3 series carrier (2.93--3.23) to run 3.23 gears. The 2.56 carrier will only go for 2.56--2.73 gears. My advice would be to find and install 3.36 gears in the 3.55 unit. With 27" tires (70 seriies 15 inch), the ratio will be 3.23 at the wheel. Good luck.


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## xconcepts (Jan 4, 2009)

geeteeohguy- I was looking through the gear ratio sticky and you said "The 3.55 (4-Series) carrier could accomodate gears from 4.88 or so down to 3.36. The 3 series carriers could accomodate gears from 3.23 down to 2.51."

Ok, so I understand I can't use the original 3.55 unless I go down to the 3.36 gears with the 27" tires. Why couldn't I use the 2.56 though, since you said its range is from 3.23 to 2.51? Is it not a 3 series carrier or can i not change it out to a 3 series?


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## geeteeohguy (Feb 2, 2008)

Xconcepts: I was WRONG on the first post! I have learned some things about these rears in the past year or so! What I found was that the really long legged ratios, like the 2.56--2.73 have really large diameter pinion gears and really THIN ring gears (to accomodate the bigger pinion). These carriers will not work with a thicker gear. They are a 2-series carrier. I put 3.36 gears in my 4-series carrier rear end om my '65 with a 4 speed, and I love it. I also pulled the stock, open 3.36 rear out of my '67 last month and installed a 2.56 Safe-T-Track. (the entire rear axle assembly). The '67 is an automatic and netted me over 19mpg on a 550 mile trip last weekend. Again, if I had your choices, I would put the 3.36 gears in your stock 3.55 rear end, and save your original gears. If you want actual 3.23 gears, you'll need to get a carrier that spans from 2.93--3.23 ratios. Good luck!!


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## xconcepts (Jan 4, 2009)

Cool thanks


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## xconcepts (Jan 4, 2009)

I do have another question before I start looking at doing this. How high will the RPM's be running at if I just change out the 3.55 to 3.36 without having 27" tires on highway and such? I have 2 sets of rims and tires, I know one is a 14" set but I'm not in the area to check the 2nd set. I will mainly be driving city when I get back to the states, but there will be some times I will get up on the highway and higher speeds.

X


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## 123jboy (Nov 15, 2008)

I don't know if this will help or not, but you may be able to estimate your rpm's with this calculator.
Differential Gear Ratio Calculator


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## Bobbyg (Jul 22, 2009)

Thanks for the link.




.


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## xconcepts (Jan 4, 2009)

So since I am half the world away from the GTO I need to ask these questions instead of finding out for myself. 

What is a good range for the RPM's be around at 70MPH? I'm not sure if i should just go with the 3.36 gears or try to find a 3 series carrier and get 3.08 or 3.23's.


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## geeteeohguy (Feb 2, 2008)

My '65 has 3.36 gears, 14" wheels, and a 4 speed. With 225/70/14 tires that are 26 inchesin diameter, it was at 3000 RPM at 70mph. Great all around gear. I recently came across some 15 inch rally ones, so I put them on the back of the car. The tires are 225/70/15: 27 inches in diameter. My RPM is now 3000 at 75MPH, or about 2850 at 70. I effectively changed my 3:36 to a 3:23 with the taller tire. My speedo is now off, too: reads about 5 mph slow. I used to run 3.55 gears in several stickshift early GTO's, and they turn too high an RPM at today's speeds to cruise comfortably for me. I was at about 3400 RPM at 70mph, maybe more. Thing is, back then, the speed limit was 55mph, so it wasn't so bad. Now, you need to be able to cruise at 75mph at least. I put a 2.56 in the back of my '67 recently, (It had a 3.36) , and man, that made a heck of a difference. Off the line suffered a significant amount, but past 30 mph, what a difference. It holds second gear until about 94mph, and flies on the freeway. My RPM at 70 is 2450, and at 3000 RPM, I'm now at 92 mph.....my old 70mph rpm!!! Fuel mileage went from 15-16mpg at 65mph to 19.5 mpg at 70-75mpg. But, this is in an automatic car. It is way too tall a gear for a stick car. For a stickshift, I think I'd like to have at least a 3.23, though. A 3.08 is not the optimum for getting it rolling from a stop. As everyone on this forum knows, I like the 3.23 ratio the best as an all around, do-everything-well gear!


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## 66tempestGT (Nov 28, 2009)

i dont think you would be able to tell the difference in 3.23-3.36 while driving if you didnt look at the tach. according to that calculator the difference is about 100 rpms at 70mph


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## geeteeohguy (Feb 2, 2008)

66TempestGT, you're correct about that. It is a miniscule difference, and with the torque of the Pontiac, you can't tell by the way the car performs. you can see it with a tach, and you might see a tenth or so quicker on a timeslip. Either way, 3.36-3.23 is a great gear for a GTO, stick or automatic.


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## 66tempestGT (Nov 28, 2009)

i agree about the torque thing. my tempest has 26" tires and would run 70mph in low gear. has to be a 2 something gear. not a racecar, but still left the stoplights ok. i miss driving it. reading this board is helping to get me motivated to get it back on the road.


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