12 bolt chevy diff

[61memo]

What would be an ideal gear ratio for my 66 burban 12 bolt diff,original 3 speed manual tranny 235/75/15 tire looking for highway cruising and not to bad city trying to get the best of both worlds ...thanks for any info

[HugoV]

I would also like to know.
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[Captainfab]

That depends on how you are using your truck. Are you using it like a truck or is it mainly a cruiser? what engine do you have? The 3.42 gears will still give you some decent pulling power. The 3.07's will be better for highway driving, but you will sacrifice some low end pulling ability. Either way, you will need to change the carrier if you have 3.73's or lower.

[LVPhotos]

http://adamsdriveshaft.com/measurements_chart

http://adamsdriveshaft.com/
Here a gear chart from the guys I use. A 3.73 with a 235/75/15 (28") tire is in the sweet spot. Any higher would improve gas milage. Lower more power. I have 295/45/18 on the rear now 28" with 3.73 posi 12 bolt and it will run all day in fourth. Its the advantage of the 5th gear overdrive to pass gas stations.

[Lugnutz65]

Originally Posted by LVPhotos

http://adamsdriveshaft.com/measurements_chart

http://adamsdriveshaft.com/
Here a gear chart from the guys I use. A 3.73 with a 235/75/15 (28") tire is in the sweet spot. Any higher would improve gas milage. Lower more power. I have 295/45/18 on the rear now 28" with 3.73 posi 12 bolt and it will run all day in fourth. Its the advantage of the 5th gear overdrive to pass gas stations.

So it sounds like you have a T5 tranny maybe?

[LVPhotos]

Originally Posted by Lugnutz65

So it sounds like you have a T5 tranny maybe?

Yes indeedee. Though an after thought the T-5 was never intened to use the torque 425+ to the rear. It is however using the factory bellhouse with motor mounts and the side engine mounts which helps the twist. The aluminum housing is its weak point.

[61memo]

Thanks for the info my 66 suburban is going to be a cruiser.what year diff can I use that i would be able to install 3.42s or 3.08 keep in mind my burban is a six lug bolt pattern thanks once again for you help and my engine is the original 283 v8

[61memo]

I forgot to add to my thread a diff that i dont have to change the carrier

[Captainfab]

It doesn't have anything to do with the year. You will have to find a '63 to early '70 12 bolt that already has 3.07 gears.

With the 283 not producing much torque, you might be better off with the 3.42 gears. I just am not familar with the 1st gear, gear ratio in those 3 speeds. If you can find a 12 bolt with 3.07's, give it a try and see how well you like it.

Originally Posted by 61memo

Thanks for the info my 66 suburban is going to be a cruiser.what year diff can I use that i would be able to install 3.42s or 3.08 keep in mind my burban is a six lug bolt pattern thanks once again for you help and my engine is the original 283 v8

Originally Posted by 61memo

I forgot to add to my thread a diff that i dont have to change the carrier

[61_FL_Apache]

I have a 3.07 in mine running 215/70/15's with a TH350 and wish I had a more taller gear in it. I plan for a taller tire next go round to get more out of it.

Between your tire size and mine, you will lose almost 6 mph just from different tires sizes.

This a tool I use to determine that.

http://www.miata.net/garage/tirecalc.html

[61memo]

Thank you all for your info as far as finding a 63 to 70 diff with 3.07 s no luck and adding 3.07s to my diff around 800 dlls but there is tons of newer rear diffs six lug bolt pattern that is why I asked. Hey 61 FL APACHE how well do you do on the freeway with your combo on your truck(3.07s and 350 tranny and 215/70/15) thanks once again for your help

[Captainfab]

The '88 and up differentials are going to be about 3" wider than the differential you have now. Plus there will be cutting and welding involved in installing one under your truck

Originally Posted by 61memo

Thank you all for your info as far as finding a 63 to 70 diff with 3.07 s no luck and adding 3.07s to my diff around 800 dlls but there is tons of newer rear diffs six lug bolt pattern that is why I asked. Hey 61 FL APACHE how well do you do on the freeway with your combo on your truck(3.07s and 350 tranny and 215/70/15) thanks once again for your help