3.07 Posi OK?

[Robert Bergstrom]

I just found out my posi is a 3.07. The stamping on the outside of the gear is 14 43 and when you do the math it says 3.07. Am I doing this right? I was hoping for a 3.75. I know freeway rpm will be great but won't be doing hours of that. Does a 200R overdrive transmission have a steeper first gear?

[WIDESIDE72]

Sounds like you have a 3.07 alright. A 700r4 has a steeper first gear than a 2004r. Do you have a bbc sbc or 6 cyl? Imo a 3.07 is too high of a gear for an od trans and it will hunt too much if you encounter hills. Iirc Gm never installed an od trans in front of anything higher than 3.42. A bbc might have the torque to mask this
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[Robert Bergstrom]

ZZ4 engine half ton fleetside. If I am not mistaken GM had a 3.07 stock with 80 and 90 V8 performance Camaros with a 700R.

[WIDESIDE72]

I meant to say trucks. The taller tire of a truck just makes the actual gear ratio higher. Try it for yourself and see what you think. I wasnt happy and swapped the 3.07 to a 3.73 in my 72 swb with a mild 350.
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[jocko]

TH350 (1-3) = 2.52/1.52/1.00
TH400 (1-3) = 2.48/1.48/1.00
2004R (1-4) = 2.74/1.57/1.00/.67
700R4 (1-4) = 3.06/1.63/1.00/0.70

700R4 has best 1st gear to make up for the 3.08. But concur with wideside, your biggest concern will be the lugging in the 45-65 mph range. If you don't do much highway driving, I'd just swap the rear gears to something a little steeper to increase your off the line perf (that sounds like where your concern lies) - or - IF you want good highway mileage also, then swap both (2004R or 700R4 and 3.42 or 3.73).

[chuckmc8]

3:08 is too tall for an OD trans IMO- 3:73 works great

[LockDoc]

Originally Posted by Robert Bergstrom

I just found out my posi is a 3.07. The stamping on the outside of the gear is 14 43 and when you do the math it says 3.07. Am I doing this right? I was hoping for a 3.75. I know freeway rpm will be great but won't be doing hours of that. Does a 200R overdrive transmission have a steeper first gear?

-
Don't know if you have seen this thread or not.

http://67-72chevytrucks.com/vboard/s...d.php?t=596343

Some good info in it....

LockDoc

[Fitz]

Originally Posted by Robert Bergstrom

I just found out my posi is a 3.07. The stamping on the outside of the gear is 14 43 and when you do the math it says 3.07. Am I doing this right? I was hoping for a 3.75. I know freeway rpm will be great but won't be doing hours of that. Does a 200R overdrive transmission have a steeper first gear?

Drive Train Specialists (DTS) and others have both the 3.48 and 3.73 ring and pinion sets for your truck. The 200R trans you have is perfect with 3.48 gears. Your ZZ4 Motor is a bit strong for the 700R4 trans which is rated at a max torque of 360 ft- lbs. If that were my truck i'd keep the 200R4 trans and switch the ring and pinion set with a 3.48 set of gears.

[cdowns]

you cant just switch gears you'd need a new posi or open carrier to do it

[Robert Bergstrom]

Thanks, have not decided on the trans yet but a 200R with 3.42 sounds good. I may go up to 3.73 if I can find a good deal on the gears somewhere. Thanks everyone for the input it has pointed me in the right direction.

[Bigdav160]

Is this a 12 bolt truck axle?

Anyone know what the case split is?
3.42 and down; 3.73 and up, is that correct?

[slow-1]

After the other recent discussion on thishttp://67-72chevytrucks.com/vboard/s...d.php?t=596343, I took my truck today to run some errands here and there.

I purposely drove it in OD and purposely drove slow. With my combo (mild 350/700r4/3.08 rear end gears, I could drive at 40 mph with no problem at all, going up hills at 40 mph, I could keep it at 40 and even accelerate without it changing down to 3rd...not stomp on the gas accelerate, just real world from 40 mph to 55 mph accelerate.

Drop down to 35 mph, it always changed to 3rd, but stayed there until I got back up to 40 mph or above.

3.73 or 3.42 may be the optimum ratio, but 3.08 is acceptable to me, your results may vary.

[Average Joe]

Personally I'd try the 3.07's. Allow me to explain my reasoning.
Your zz4 makes very good torque at a lower rpm than many other cammed up small blocks. Look at the cam specs. It is very short on duration but long on lift. Something like 208 and 212 @ .050 IIRC. This makes for a lower torque curve. Plus a 10 to 1 comp. ratio with aluminum heads makes for efficiency as well as power. However detonation could be an issue on a hot day or poor fuel. You may have to play with the timing to get it dialed in. Acceleration would definitely be better with either a 3.42 or 3.73. I presume the reason you went with an overdrive was to improve hwy MPG and get lower rpm. This combo should attain that. Board member gchemist runs the same combo and claims close to 20mpg hwy. Now if memory serves he is in the flat midwest but so is the willamette valley. I personally would try them first before spending lots o'cash on a new settup. Just one Average Joes 2cents.

[lha1992]

Originally Posted by WIDESIDE72

Sounds like you have a 3.07 alright. A 700r4 has a steeper first gear than a 2004r. Do you have a bbc sbc or 6 cyl? Imo a 3.07 is too high of a gear for an od trans and it will hunt too much if you encounter hills. Iirc Gm never installed an od trans in front of anything higher than 3.42. A bbc might have the torque to mask this
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GM built lots of 88-98 GMT-400 trucks with a 350FI, 700R4 and 3.08s. My 89 came like that from the factory with 235-75R15 tires.

I just rebuilt and built up the 700R in my truck and getting ready to switch to 3.73s in it.

[LONGHAIR]

Originally Posted by WIDESIDE72

Iirc Gm never installed an od trans in front of anything higher than 3.42.
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Not true at all. In fact, in half ton trucks, you would be less likely to find one lower than that. In the early years of the OD transmissions they used gears as high as 2.56......in a 4x4 no less. They were extreme duds, made worse by bigger tires.
I replaced dozens of 2.73s and 3.07s back in the day.

[Fitz]

Originally Posted by cdowns

you cant just switch gears you'd need a new posi or open carrier to do it

Since the breakpoint for resizing the carrier is 3.08 small and anything shorter, large no matter what he does except keep the 308 means changing the carrier.

[Robert Bergstrom]

I have series 3 posi 12 bolt. My understanding 3.42 is the steepest gear that came from the factory for a series 3, but 3.73 are sold now that have the thicker gear and will work in a series 3. Decisions, decisions...