Gear ratio available in 1970 on 4X4s ?

[hemi43]

Just picked up my 70 Blazer project, and was disappointed to find a 3.08 tag on the front diff. The front is a Dana 44 which was easy to determine, and I assume the rear is a GM 12 bolt but have not confirmed.
The truck sits on 33X12.5 tires, and because I plan on installing an overdrive transmission, 3.08 will be way too stiff.
Did they really put 3.08 gears in the 70 Blazers? I thought for sure it would have a 3.73 at minimum.
Also, if I decide to change to something like a 4.10 gear, will the carrier have to be changed?
Thanks

[RIDE-RED 350r]

Well, without digging up old options for Blazers in 1970 on google, I don't think it is at all out of the question that they would come with 3.08 gears.

Think about it, TH350 3spd auto, or a 4spd w/granny low were your transmission choices for a K-5 or K-10 at that time. On the stock tires sizes that came on them, they would be turning some pretty serious RPM at 60mph with 4.10 gears. 3.73 might even have been a bit much.

I have a PDF (over 100 pages if I recall) of what seems to be a book of all specs and options for GM trucks in 1978. And if I recall, 3.08 was std equipment on many packages in that year.

As to your question about re-gearing and your carrier: yes, you can go with 4.10 but you will need to get a 4.10 gear set with a "thick" ring gear.

I'll explain: 3.42 and numerically lower gear sets were used on a series 3 carrier. Whereas 3.73 and numerically higher were used on series 4. Series 3 or 4 simply refers to ring gear distance from the pinion. The fatter pinions on numerically lower gears obviously need a bit more room between them and their ring gears. So the ring gear mounting flange on the carrier is located farther away from the pinion. Conversely, series 4 carriers place the ring gear mount flange closer to the pinion due to the smaller diameter pinion gears found in numerically higher gear sets.

You can always go numerically higher with your gears if you have a series 3 carrier. You just need a gear set with a "thick" ring gear if you choose 3.73 or numerically higher. However, you cannot run 3.42 and numerically lower on a series 4.

Hope I explained that clearly.

[RIDE-RED 350r]

This should help you. This is where I found the pdf for the 78 trucks. It's a long read, but the info you seek should be there.

And looky here... I was wrong. That link states that 3.73 was std equipment for a Blazer in '70, depending on engine, transmission and whether it is a C or a K.

https://gmheritagecenter.com/docs/gm..._Full-Size.pdf

[hemi43]

Thanks Ride-Red !!
As you can see from my user name, I've owned Mopars all my life so this Blazer is all new territory to me. I knew that different carriers were needed for different gear ratios to make up for the smaller pinion diameter, but I learned today that someone actually made thicker ring gears to make up for it. This will save me a lot of money seeing as just a ring and pinion set and bearings will be needed.
I checked with both Yukon and Richmond and thick gears are available for both the Dana up front and the 12 bolt in the rear.

[Mike C]

My Jimmy had 3.07 in it as well. In the 1/2 tons it was either 3.08 or 3.73. No need for and OD with 3.07 at least....

Good time to score a set of 3.73 8 lug axles and not have to deal with broken 12 bolts in the future.

[hemi43]

I think I will opt for 4.11s
The tires are 33" tall, and I will be putting in a 5.3 with a 4L60E. At 60 MPH that will put me in the 1800 RPM range which is perfect.

[special-K]

I had a '90 Blazer 4spd with 3.08s and 33/12.50s. It rolled on down the road with the overdrives just fine. 75 mph was a breeze. But if you're swapping in a computer your plan is the easy way to go.

[hemi43]

I guess i forgot to mention that I plan on towing a 27 foot travel trailer when I go to shows. The 4L60E is not the strongest transmission, so the 4.11 gear can only help it survive a bit better.

[D.PASSMORE]

That is a whole lot of trailer for a small Blazer. I have 19' open car hauler trailer (ELEC Brakes) that I pull with my 72 Blazer (Mild build SBC 350, SM465, 3.73 gears (posi rear), 2.5 Skyjacker lift, 33x12.50's) and its a daunting task to pull the trailer w/ another vehicle loaded up. Plus if your suspension isn't super solid, you will feel the death wobble around 55 mph. You might want to ensure it can handle that load before you dump a lot of money into the Blazer and jeopardized your safety. Just saying...

BTW, GM generally put 3.08 gears in automatic transmissions and 3.73 gears with standard transmissions.

[hemi43]

Originally Posted by D.PASSMORE

That is a whole lot of trailer for a small Blazer. I have 19' open car hauler trailer (ELEC Brakes) that I pull with my 72 Blazer (Mild build SBC 350, SM465, 3.73 gears (posi rear), 2.5 Skyjacker lift, 33x12.50's) and its a daunting task to pull the trailer w/ another vehicle loaded up. Plus if your suspension isn't super solid, you will feel the death wobble around 55 mph. You might want to ensure it can handle that load before you dump a lot of money into the Blazer and jeopardized your safety. Just saying...

BTW, they put 3.08 gears in automatic transmissions and 3.73 gears with standard transmissions.

Yeah, the short wheelbase is definitely a concern. I will be starting a frame off restoration on it with plans of having ready for next year. As far as a "solid" suspension, everything will be new. I do have a load leveling hitch as well as an anti-sway bar, so I have my fingers crossed that this will be sufficient. The dry weight on my 27 footer is only 4000lbs which would put it much lighter than your car trailer and load. Time will tell, and if it proves to be too much to haul, I have to revert back to a smaller popup trailer.

[Mike C]

Originally Posted by D.PASSMORE

BTW, they put 3.08 gears in automatic transmissions and 3.73 gears with standard transmissions.

Don't know if it stayed the same 70-72, but I have a '72 automatic Jimmy that we are going to restore for my son that is 3.73 and my SM465 Jimmy was 3.07. So YMMV in addition to your ratio!

[A1971Blazer]

My 1971 had SM465 manual with 3.08 gears....... I assume they did this because the factory tires were relatively small in diameter and the 465 had the "granny" first gear to make low end grunt when you needed it.

My swap is 5.7L LS1/4L60E/NP241.......I am re-gearing to 4.11 with truetrac rear 12 bolt and open front D44

[68panelman]

My 1970 Blazer also has 3:08's, Original 350, 4spd, 35" tires.

[swamp rat]

My 72 has 4:10's with a 350/350 combo.