3:07 Rear End
 

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? :chevy:

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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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

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

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.

Re: 3:07 Rear End
 

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. :lol:

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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!

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

3.08

Re: 3:07 Rear End
 

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.

Re: 3:07 Rear End
 

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..... :D

Re: 3:07 Rear End
 

I believe truck is 3.07, car 3.08. Longhorn Man used to correct everyone on here when the subject came up as it was a pet peeve of his :lol: I think he has close to every book GM ever published and was a wealth of knowledge. Search some of his old posts and you’ll find it.

Re: 3:07 Rear End
 

Using the numerical ratios is a recipe for errors anyway, so you're better off with the actual tooth count, which is how I keep them straight. Plus, when you look in the parts book, that's how they identify them.

For example:

A 3.08 gearset is 12/37 which is 3.0833333…
A 3.07 gearset is 13/40 which is 3.07692307…

You could have other setups as well, like in a larger rear end you could have a 14-43 setup which would give you 3.0714

Re: 3:07 Rear End
 

I have a vortec 350 with a lot of low end torque with a th350 and a 307 rear. It works out great for me. Torque to get it going and gears for highway driving.

Re: 3:07 Rear End
 

3.07 can work. Needs to be a V-8, preferably a 350 or larger. I drove my 67 Bel Air with a 283, TH350 lockup and 2.73 gear for 175,000 miles. I experimented for try for some fantastic MPG number. 20 MPG was all I got. If I were to do it over it would be a small block 400, TH350, maybe a 3.08 gear. screw the absolute MPG numbers. A heavy vehicle LIKE a pickup needs gearing to make things easier for the engine.

I'm not a proponent of O.D. transmissions. My 67 and 68 pickups will both have 3.73 gears when they get back on the road. I will compromise on the side of power the rest of my life. Needs to be at least a little fun to drive. My Bel Air ran great, just wasn't any kind of a powerhouse...

Re: 3:07 Rear End
 

My 383 runs great with a 700R4 very powerful truck

Re: 3:07 Rear End
 

Im in need of a set of 307 gears:metal:

Re: 3:07 Rear End
 

I guess I somehow got it reversed at some point. I could swear we went over this time and again and the correction was trucks were 3:08. I mean the threads from when Andy was here. He was the book smart knowledge and I was the knowledge from personal experience and conversing with the other 67-72 truck owners since new. And 3.08 was always the common knowledge reference, as I recall. I better stop recalling :lol:. Honestly, I never paid heed to any ratio taller than 3.73. In fact, 90% of my trucks have had 4.10/4.11 and lower gearing for the grunt. I ordered my '92 K3500 with 4.10/5spd and loved it. I got a '95 with same powertrain except 3.73s and hated it off the line. Was looking to go to 4.10s before it lost it's life. Not until the dollar has gotten so hard to spread that I've given a hoot about better hiway gearing to save on fuel. I routinely drove my 4.10 equipped trucks down the interstate at 70-80 mph and never gave it a thought. In fact, I love that sound! The engine in my '72 GMC I rebuilt in '91. It was in my Cheyenne Super K/20 and in '04 I swapped it into the Sierra K2500. That same motor, still healthy and strong, has countless hours of the RPMs that 4.10s and that speed produces. I also had 4.10s in the '71 Custom Camper I had for 18 years and traveled as far as Grand Canyon in it at hiway speeds. I have that uncracked (except no-lead head job and timing chain) original still healthy motor on a stand waiting for it's next assignment. O/D definitely adds life to a vehicle...if you don't make up the drop in RPM with higher speed driving. But in my experience, driving at 3 grand plus never hurt a thing. And if I want to save on gas I just drive slower.

Re: 3:07 Rear End
 

I'll tell you the ones I know, perhaps others can fill in the empty spaces:

GM 10-bolt car: 3.07 12/37 3.08333
GM 12-bolt car: 3.08 40/13 3.07692
GM 12-bolt trk: 3.08 40/13 3.07692

So that leads me to believe the cars and trucks are the same ratio, at least they have the same tooth count. I'm getting the truck numbers from this Yukon gearset for the GM12T-308:

https://www.summitracing.com/parts/yga-24459

Re: 3:07 Rear End
 

I have a 1970 TDE coded rear in my shop and it comes back to a 1970 3.07 open rear. Here's a thread talking about the same TDE coded rear.
http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Re: 3:07 Rear End
 

Don't make a lot of dif [no pun] 308/307 , Only if counting to replace one or other,[ring or pin]......just get in the boat and row,no need to count the waves !:haha:

Re: 3:07 Rear End
 

The 3:07 is a 3:08, just as the 3:08 in many cases is referred to as a 3:07. Car, truck, standard or automatic, it doesn't matter, they are the same. Davepl pretty well clears it up with the varying tooth counts, there is no such thing as an exact 3:07 or 3:08. I think the "three-o-eight" won notoriety because it's easier to say than "three-o-seven".
Tim is correct in that although the eight lug Dana 44's are referred to and matched with 4:11 Dana 60's or 4:11 14-bolts, it's actually a 4:09. No gear set is dead nuts, you can call it what you want, but the terminology will forever be a 3:08, it's just easier to say. Besides, a 307 is a motor not an axle.
Harpo your correct in that it dont make a difference, and incorrect because you never change out just a ring gear or pinion. They are married sets lapped together from the foundry, when one goes the other must go with it.
LockDoc is also correct in sharing his experience with OD transmissions, six cylinder motors and high geared axles. Cant compare a 5000 LB panel truck to a 3000 LB later model unibody. GM did not put 3:08 axles behind six cylinder trucks for obvious reasons.

Re: 3:07 Rear End
 

What motor? Thats the decisive factor in this equation. Cant really compare a 69 straight six pulling a truck to later model V6's or V8's in lighter packages..."just sayin" ;)

Re: 3:07 Rear End
 

Corporate rears and Danas ran ratios right next door to each other. When I blew a ring & pinion in an HO52 I found there was a 4.56 and a 4.57 in the same year. The 4.57 GM listed as "non-Dana" and that was the ticket.

Like I said before, the ID tag on the front of my '72 K/20 Highlander read 4.09. The difference between that and the HO52's 4.10 doesn't make enough difference to matter and if it had a Dana 60 that would be 4.11. So the trucks care a lot less about this than we do :lol:

Re: 3:07 Rear End
 

I've got a Dana 44 out there in the shop I'm rebuilding for a kid that has that same tag. Has 4:09 and the tooth counts on it, he's going to run that with a 14-bolt rear with a 4:11 in it...I learned the hard way a long time ago, it's better to run a tic tighter in the front than in the rear...;)

Re: 3:07 Rear End
 

IMO you would be better off with your 3.73 gears and OD. My truck with 3.08 and 700r lugs the motor at 55mph.But it will run the speedo back to zero.According to the gear charts with 29 inch tall tires,3.08 gears.at 5500 RPM in OD you will be going 212MPH. I read this in a old Street Rodder mag years ago.