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Old 02-21-2012, 06:09 PM
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Change 2.73 rear to 3.55?

Hi everyone, I'm new here. I have a 79 firebird formula with a 2.73 posi. Someone has installed what appears to be a vortex 350 with a turbo 400. It has the 225/70/15 tires on it. Problem is, has good power but doesn't smoke the rear tires, and isn't that why we like hot rods? I'm thinking of going to a steeper gear, not a daily driver but want it street able. Also, can the existing carrier take gears in the 3.55 range? I know an overdrive trans and steeper gears would be ideal, but don't want to sink a ton into this car.
Thanks,
Bill

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Old 02-22-2012, 06:47 PM
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Quote:
Originally Posted by billmillikan
Hi everyone, I'm new here. I have a 79 firebird formula with a 2.73 posi. Someone has installed what appears to be a vortex 350 with a turbo 400. It has the 225/70/15 tires on it. Problem is, has good power but doesn't smoke the rear tires, and isn't that why we like hot rods? I'm thinking of going to a steeper gear, not a daily driver but want it street able. Also, can the existing carrier take gears in the 3.55 range? I know an overdrive trans and steeper gears would be ideal, but don't want to sink a ton into this car.
Thanks,
Bill
Need to figure out which diff you have. If you have the 7.5 you will need to get a "Thick" ring gear once you go lower that the 308 ratio.

If you have the 8.5 your carrier will accept the ratio your wanting.

So either way it can be done.
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Old 02-22-2012, 07:13 PM
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Originally Posted by prostreet6t9
Need to figure out which diff you have. If you have the 7.5 you will need to get a "Thick" ring gear once you go lower that the 308 ratio.

If you have the 8.5 your carrier will accept the ratio your wanting.

So either way it can be done.
If its an 8.5 its either a 2.73+ carrier or a 2.73- carrier...since it has the 2.73 gears you will need a ring gear spacer to get the 3.55's to work

Went through this with my 78 trans am, 8.5 rear end...had to put the spacer on it.
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Old 02-22-2012, 08:04 PM
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Originally Posted by BigRoy1978
If its an 8.5 its either a 2.73+ carrier or a 2.73- carrier...since it has the 2.73 gears you will need a ring gear spacer to get the 3.55's to work

Went through this with my 78 trans am, 8.5 rear end...had to put the spacer on it.
If its a 273 carrier you can put 273 and lower gears on the carrier. NO spacers needed on the GM 8.5.
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Old 02-22-2012, 09:41 PM
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yes, Bill your certainely have a 8.5 diff. and the 2.73 and numerically higher gears (3.08, 3.42, 3.73, 4.10 and up) will fit in the carrier.

if you want to really light up the tires ,put a 4.10 in there. With 28 in. tires you should cruise at 3000 rpm or close at 60 mph.


for reasonable fuel compsomption you should put 3.73 and have good punch when you put the pedal to the metal.

thank you!

MICHEL
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Old 02-23-2012, 05:29 AM
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I'd keep the 2.73 and put a lower first and second gear set in the tranny....
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