Gears for a GM 8.5" rear?

[Samckitt]

It is an 86 GN rear that is behind a T56 trans. It still has the factory gears of 3.42:1. I have been debating on changing to a 3.90, 4.10. or 4.11 rear. First off what is your suggestion & second will all of these gears fit on the same carrier or will I have to change carriers for the 4.10/4.11?



Thanks

[GBodyGMachine]

With the six speed I would go with a 4.10. You have plenty of OD to make up for the steep gear. I think you will have to change carriers though.

Jeff

[big gear head]

The 8.5 rear end uses a 3 series differential for all gears from 2.73 to 5.13, so you are good with the one that you have. The 4.10 would be good with this transmission, but you need to have a good drive shaft. If you are using the stock drive shaft then you will probablu have a vibration at higher speeds. This is because of the high drive shaft RPMs with the overdrive transmissionand the lower rear end ratio. The drive shaft will distort at speed causing the vibration. Most of the time a larger diameter drive shaft tube is needed for this combination.

[TnBlkC230WZ]

You don't say which t-56. If it is the 2.66 first gear. I'd recommend the 4.11s. If it is the 2.97 first gear. I'd stary with the 3.42.

[Samckitt]

It is from a 95 F body & has the 50% 6th gear, not sure what the first gear is. Maybe someone can tell me.