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#1
09-07-2005, 03:52 PM
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Rear Gear Help
I hate to ask because I know so many people have asked the same but here are my specs. 68 GTO with a 428 headers, Carter AFB, Holley Dominator intake, Roller Rockers, Ultradyne Cam(223 intake/232exaust). Turbo 400 Tranny (Mild Shift Kit). Car now has a 2.78 open rear(I can't understand why previous owner did that).
QUESTION: I am buying a Eaton or Auburn Pro Posi Unit. CONSIDERING MY ENGINE AND TRANNY SPECS, Which gear would be better for acceleration 3.55 or 3.90? I know highway driving hurts with the 3.90 but my question is acceleration. Thanks to all!!! |
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#2
09-07-2005, 04:27 PM
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3.90 for acceleration, but if you want to drive it any amount I would go with the 3.55. I had a 4.56 in my 68 with turbo 400 and 3500 stall and i couldn't drive it anywhere. I now have 4.11 and a 700R4 overdrive but haven't driven the car yet.
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#3
09-07-2005, 04:35 PM
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Either gear ratio would be fine. If you are planning to bolt up some slicks you might want to think twice about the 8.2 rear end though. They are notoriously weak. My 455 shredded mine with street radials!!
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************************************* 1968 Lemans. 37,000 original miles. GTO clone. 462ci/KRE 290 heads. UltraDyne 280/288 Solid/850 Qjet by Cliff/Performer RPM/TSP 9.5" in TH400/8.5" 3.42 gears/3950# Race weight/12.58@106 at Bandimere speedway high altitude |
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#4
09-07-2005, 05:44 PM
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Height of tire can affect the gear ratio you choose as well. Stock converter? 26" tire 3.55. You will see a huge improvement over the 2.73 with a 3.55. Get the Eaton over the Auburn.
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#5
09-07-2005, 06:40 PM
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Tire size is 235/60/15 25 inches. I plan on buying 245/60/16 about 1/2 inch taller. The stall on the car is a 2800. No slicks, just want to drive her on the street. I just want the car to accelerate faster. Darn with 8MPG i should at least take on a these rice bangers with turbos. LOL!!
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#6
09-07-2005, 06:41 PM
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Sorry, typo, I plan on buying 245/60/15.
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#7
09-07-2005, 08:56 PM
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You should definitley consider if you will cruise in this car. A 235/60/15 is closer to 26" diameter. With a 26" tire, the cruising RPM at 75 mph will be rear gear ratio times 100, so 3550 with the 3.55 and 3900 with the 3.90. It's noisy but most people can cope with 3500 but 3900 is really buzzing and very unpleasant.
If you are just cruising round town, and want to squeal away from every light, and break traction on the shifts, the 3.90 will work well, but that can get pretty tiring after a while (believe it or not!). With the broad torque band that cam should give, I think you will like the 3.55. |
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