3:07 Rear End

[old51sedan]

My 69 C-10 with 6 cyclinder and 3 speed automatic has a 3:73 gear which I understand is pretty much standard. I understand that the 3:07 & 4:11 were options. I would like to install a 200-4R and possibly a 3:07. If a truck had a different rear gear from the Factory it be listed on the SPID on the glove box wouldn't it? I know of a few trucks in the bone yard, most are not in a position as to where I can turn the rear axle to determine the ratio. Just how common is it to find a 3:07 rear end?

[davepl]

If you put in a 200-4R I don't think you need anywhere near a 3.07 unless you plan to run at Bonneville! The overdrive is .67 or so, which would make that 2.04:1 rear end!

I'm running 4.10s and when the overdrive kicks in it's like a 2.75:1 on the highway and that's about perfect.

Am I missing something? I'd hate to see you wind up with an unusably tall gear.

[special-K]

Trucks had 3.08s, not that it's a big difference. Most C/10s with automatic transmissions came through with 3.08s. I'd do the overdrive transmission first, then see what you think.

[LockDoc]

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davepl is correct. A 2004r and a 3.08 rear gear do not play well together. The ideal gear for the 2004r is the one you already have. A 3.73

I tried the 2004r and 3.08 in my 69 Suburban and it put too much stress on the trans when it was locked up in OD. It overheated and blew trans fluid out the front seal..... If you left it in 3rd gear it worked OK but that defeated the purpose of the OD trans. Put a 3.73 in it and no more problems.

LockDoc

[RustyBucket]

My '71 GMC stepside has a mild 350 & a 700r4 with a 3.08 rear gear. The 700r has a .70 OD ratio. The steep 3.06 first gear is responsive enough. It shifts itself into OD at about 50mph and doesn't feel lazy. I don't have a tach in the truck. It runs 70+ on the interstate happily. Have 275/60/15 tires. Don't know how your 6cyl. would like it. I think my gas mileage suffers because the engine is running below it's powerband most of the time and the converter is non-lockup.

[Daves70]

3.73 seems to be the sweet gear ratio for the o/d transmissions. With that you can have it all. Decent power and lower rpm at cruising.

[special-K]

3.42s from a later 12-bolt are a great ratio for O/D. Better on the hiway than 3.73 with a loss of grunt you probably won't notice. Better with O/D than 3.08s and bottom end.

[rsavage]

I put a 200 4R in the 70 Burb I had with its stock 3.07 gears ( stamped 43/14 = 3.07). It shifted into OD at about 45 mph which dropped the rpms down to about 1000. Couldn't use it around town so kept it in 3rd. Worked OK on the highway. Getting the TV cable correctly adjusted is critical to transmission longevity. Bow Tie Overdrives makes some carb linkages for quadrajets and holleys but last time I checked they had nothing for a six cylinder set up. I don't know how much highway driving you do, but maybe just changing out the rear end gears to a 3.07 would work for you. The transmissions are finicky relative to TV cable and don't last long if adjusted wrong. Not a cheap conversion either.

[Jrainman]

Originally Posted by LockDoc

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davepl is correct. A 2004r and a 3.08 rear gear do not play well together. The ideal gear for the 2004r is the one you already have. A 3.73

I tried the 2004r and 3.08 in my 69 Suburban and it put too much stress on the trans when it was locked up in OD. It overheated and blew trans fluid out the front seal..... If you left it in 3rd gear it worked OK but that defeated the purpose of the OD trans. Put a 3.73 in it and no more problems.

LockDoc

I run a 3.08 with a 200 4r in my 68 with no issues, I dont understand when you say they dont play well together , a lot of Chevys' Buicks' and Olds came in this configuration in the 80s and 90s even the 1990 Caprice Police pakage came with a MW9 (200 4r ) Trans and 3.08 rear diff.

[LockDoc]

Originally Posted by Jrainman

I run a 3.08 with a 200 4r in my 68 with no issues, I dont understand when you say they dont play well together , a lot of Chevys' Buicks' and Olds came in this configuration in the 80s and 90s even the 1990 Caprice Police pakage came with a MW9 (200 4r ) Trans and 3.08 rear diff.

I was just posting my first hand experience. It took me two refills of trans fluid before I figured out what was going on. If a person doesn't use the lockup or has a trans oil cooler it might not be a problem, or maybe it was just the bigger vehicle I installed it in, not sure, but no more problems after I changed to the 3.73 with the same transmission.

I don't know anything about the '80's and '90's cars. Maybe they had trans oil coolers. I would bet the police package did. I basically post that info in the 2004r threads I come across so if people have the same problem I did they will know what the cause is..... As usual, everyone is free to try whatever combo they want.

LockDoc

[demian5]

Yeah I love my 3.07 with my 350/350 and 31" tires...

[special-K]

I think truck and car requirements for gearing differ due to weight and general use of the design. To get the same power to weight ratio requires different horsepower and gear ratio for same affect. That's why GM didn't even offer 3.08s or 3.73s in 3/4t trucks

Originally Posted by rsavage

I put a 200 4R in the 70 Burb I had with its stock 3.07 gears ( stamped 43/14 = 3.07)

Like I said, not much difference. 43 divided bt 14 comes out a bit more than 3.07 and I guess GM decided to round it up for trucks because that's what GM calls them...3.08s. Just keeping the terminology the same as GM is the only reason I brought it up. The Dana 44 in the front of my 4.10 rear K2500 is marked 4.09 on the ID tab. The Dana 60 rear is actually 4.11.

[yuccales]

Originally Posted by davepl

unless you plan to run at Bonneville!

Made me smile a bit. My '75 GMC Sprint came with the "economy rear axle ratio" which was 2.56. That thing loved to stretch it legs! Flat towed a VW Bug once, don't think that 350 trans ever got out of second gear.

[old51sedan]

Thanks for the replies. I have the 200-R4 all rebuilt and ready to go in. Was going to lower the truck and figured that would be the time to change the rear end. I may just leave the 3:73's in until after I change the trans. and see how she does.

[davepl]

Definitely keep the 3.73s... if you had 3.08s, we'd be telling you to swap for 3.73s anyway!

I'm restoring a '69 2+2 and while it has 3.31s because it's a manual, I saw that the default axle for the LS1 427 (335hp) with the TH400 was the 2.56 rear end. That's a steep setup! But 3.08s with OD would be significantly steeper than that even.

Assuming a .7 OD you'll wind up at 2.61:1 which is darned close to that 2.56:1 number!

[RustyBucket]

Originally Posted by davepl

Definitely keep the 3.73s... if you had 3.08s, we'd be telling you to swap for 3.73s anyway!

I'm restoring a '69 2+2 and while it has 3.31s because it's a manual, I saw that the default axle for the LS1 427 (335hp) with the TH400 was the 2.56 rear end. That's a steep setup! But 3.08s with OD would be significantly steeper than that even.

Assuming a .7 OD you'll wind up at 2.61:1 which is darned close to that 2.56:1 number!

Dave, BIG difference in torque between old51sedan's 6cyl. C10 and your 2+2

[special-K]

Figure the only 8-lug truck available with 3.54:1 were big blocks. 6cyls typically ran lower gearing than V8s. Gear ratio/weight/torque is the combo to consider

[garyd1961]

Originally Posted by special-K

Trucks had 3.08s, not that it's a big difference. Most C/10s with automatic transmissions came through with 3.08s. I'd do the overdrive transmission first, then see what you think.

I'm pretty sure I looked up the codes on both of my 1970 C10 rear ends and they came back to 3.07. I found several different links to gm charts and all came back to 3.07. Here's one chart but the one I saw before had a lot more info on it.
http://www.348-409.com/differential.html

[special-K]

That's cars. GM called the car ratio 3.07. It's really no big deal. I was just advising as to what the manufacturer referred to the ratio as. A 350 isn't actually 350 cu in but that's what Chevy called them. I didn't mean to hijack this thread

[trac209]

Originally Posted by special-K

That's cars. GM called the car ratio 3.07. It's really no big deal. I was just advising as to what the manufacturer referred to the ratio as. A 350 isn't actually 350 cu in but that's what Chevy called them. I didn't mean to hijack this thread

I’m pretty sure you have that backwards,the cars had 3.08 and trucks had 3.07

[bbrriiaann1973]

I have a 383/700R4 with 3.73 gears and it works great. Nice balance between city driving and highway.

[special-K]

Originally Posted by trac209

I’m pretty sure you have that backwards,the cars had 3.08 and trucks had 3.07

I know I don't and that chart supports it. All my GM literature refers to the truck ratio as 3.08:1 including the parts book. Can we move on now? Let's focus on what I typed about the three important factors to consider, gearing/weight/torque. That's what all this boils down to. My purpose here in this thread is to help.

[bbrriiaann1973]

3.08

[garyd1961]

As I said I have two 1970 Chevy truck rears that the codes come back to a 307 gear on every chart I can find.

[Rich69shortfleet]

I pulled a set of truck 12-bolt 3.07/3.08 gears out of my truck to install 3.73 gears. I have a stock (rebuilt) TH350. The 3.73s made it a MUCH better tow vehicle (1000lb trailer, 3500lb car). But if I had an overdrive trans with a deep first gear already the 3.07/3.08 would be a good choice.

Now if anyone needs a set of 3.07/3.08 gears in good shape I'm sick of tripping over them in my shop.....