Gear ratio upgrade

[PhatNBad]

Hi all....I have a 70 with a 12 bolt, 2:73 rear. Can I upgrade to 3:73's without having to change the carrier? What carrier do we have in our cars? Im looking in my Summit Catalog and they list 2 3:73's for a GM 12 bolt. One of those says "fits 4 series carrier" next to it, and the other says nothing. These are Richmond ring and pinions. Also, whats the general opinion on 3:73's? Too much? I do plan on swapping in a 200-4r also. I know with 3:73's and the th-350, highway driving and gas mileage will take a big hit. Thanks for the input.

[Guest]

I put in 3;90 s and a posi all at once,what a difference! but, I have a 454. Alot of people say 3;73s are the best to go with but it realy depends on what your plans are. I street race mine so thats why I went with what I have.

[PhatNBad]

Thanks for the reply Kebi.....Im just running mine on the street, and an occasional cruise on the highways, so I want to keep it streetable but the 2:73s are killing me outta the hole when I jump on it a little. My thing is I dont know what the "Carrier" reference is, or which carrier we have in out Montes.

[71_LES_402]

I don't recall the difference in carriers. A friend of mine went through it once, but I couldn't truthfully answer your question.

I am real happy with my 373's and want to put an overdrive tranny in mine as soon as the money appears. HA!HA!

Les

[wallaby]

I'm not a gear expert, but I believe that the "4 series carrier" is one that is used with gear ratios 4 and larger. So 4:11 gears for example would be mounted on 4 series carrier.

If I were you I'd do some calculations to find out what kind of cruise RPMs you would have with that overdrive trans. Tire size is important, too...so gather the information and start plugging different gear ratios into the formula to see what works best for you.

[Guest]

Wallaby you are right on the tire size. I have 215 65 15 and I definetley need to go higher according to all the other posters on here. My car flies off the line but I lost alot of speed on the highway.

[Guest]

215 65 15....WOW is there such thing as traction????LOL

2-series - 2.73 and numerically lower gear set for 8.5" carriers, 3.08 and numerically lower gear set for 7.5" carriers.

 

3-series - 3.08 and numerically higher gear set for 8.5" carriers, 3.23 and numerically higher gear set for 7.5" carrier

Not sure about 12 bolts or other 8.? sizes

[72MC]

If you have a one leg carrier right now and want posi, you will have to change carrier. Now, this next point has arguements on both side, if you want to go to 3.?? gears, you should go with a series 3 carrier. Some people argue that all you need is a shim and you can use a series 2 carrier. I dont like that way of doing it. I have an original GM series 2 posi carrier and from what I understand they are not readily available. However, series 3 posi carriers are everywhere, brand new or used. I prefer Eaton posi carriers. Series 4 carriers and gears are the same as above.

[big gear head]

The 12 bolt uses a 2 series differential for the 2.73 gears. You will need a 3 series to use the 3.73 gears. The Eaton posi is the best for street use. It is rebuildable and smooth operating. You will need to get a good installation kit to go with the gears. It will have all of the shims and small parts that you will need to do the swap. The gear that is listed in Summit with no note is for the 3 series differential. If you are going to install the overdrive transmission then the 3.73 will be fine. If you keep the TH350 then you might want to consider the 3.42 gears.

[Guest]

I would soooo go with a 200r4. You can not go wrong with that sweet .69 o/d. OH MY LORD! that means you can drop a 4.10 gear in the back and still cruse on the highway! ESPECIALLY since it has a lockup torque converter! switching to 3.73's along with a 200r4 will get you aproxamitly to the same rpm@ 75 as you are at right now at 60 (er, something like that)