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Apr 17, 2008 | 10:01 AM

BritewhiteC3

Thread Starter

Instructor

Joined: Feb 2008

Posts: 140

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From: Tulsa OK

Gears

I'm considering changing my rearend gears from a 3.08 to 3.55. I have a mild built SB i.e. vortec heads, Edelbrock 600, true duals w/Hedman's 2.5" mild cam. Would I get a "seat of the pants" difference, and how will highway mileage compare?

Thanks Joe

Apr 17, 2008 | 10:28 AM

GTR1999

Tech Contributor

Joined: Aug 1999

Posts: 15,114

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From: Connecticut, USA

355 were the performance gear for auto's. You'll feel it, maybe not as much as you expect but the rpm will be higher on the highway, about 300 RPM. Gas mileage cruising will go down but I doubt you'll see much difference around town.

Apr 17, 2008 | 12:17 PM

jnb5101

Le Mans Master

Joined: Jul 2006

Posts: 8,117

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From: charlotte north carolina

the 355s will keep your rpms in the working range of your mild cam. go for it.
jeff

Apr 17, 2008 | 05:25 PM

imbill

Navigator

Joined: Jun 2005

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I changed mine from 3.08 to 3.55 and it feels good and the rpms arent all that bad. Your setup is similar to mine and I love it!!! The true results will be seen at the track I hope.

Apr 17, 2008 | 06:04 PM

Amelio

Burning Brakes

Joined: Oct 2007

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What trans do you have? How tall are your rear tires? What do you want out of the car? These are a few questions I ask myself before something like that. It all depends on what you want and how you want the car to feel and act. I personally like cruising around town not too much hwy and of coruse I drag my car. I have 3.73 but I will be putting in 4.33 because of the engin I just built.

I Have always been a fan of 3.73 especially with an OD trans and a
26" tall tire I think it is a great combo

If you go from .08 to .55 you will feel it and your not going to have a huge difference in final drive for gas milage. IMHO 3.73 will make the biggest difference seat of the pance and real preformance for a mild sbc. You cruising rpms will increase but nothing drastic, in your final drive or 1:1 60mph is usually about 3000rpms which makes the car spunky even from a 40-50 mph roll.

Hope that helps....good luck with your choice

Apr 17, 2008 | 06:41 PM

BritewhiteC3

Thread Starter

Instructor

Joined: Feb 2008

Posts: 140

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From: Tulsa OK

Thanks for the replies.

I'm running a TH400
My tires are Firestone Firehawk Indy 500's P255/60 R15
An OD 700R4 may be in my future, A friend wants to trade me but it won't be until this winter.
I like to cruise on the highway but want a little more off the line, so I figured the 3.55 is a good middle of the road trade off.

Apr 17, 2008 | 07:41 PM

Glensgages

Race Director

Joined: Jan 2003

Posts: 15,172

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From: State of Confusion

Quote:

Originally Posted by BritewhiteC3

I'm running a TH400
My tires are Firestone Firehawk Indy 500's P255/60 R15

I like to cruise on the highway.

You can use this for a 'ball-park' figure between RPM & MPH, calculated with your above data.

3.55:1 gears
2100 RPM = 42.3 MPH
2400 = 48.3
2700 = 54.4
3000 = 60.4

3.73:1 gears
2100 = 40.2
2400 = 46.0
2700 = 51.7
3000 = 57.5

4.11:1 gears
2100 = 36.5
2400 = 41.7
2700 = 47.0
3000 = 52.2

Apr 18, 2008 | 12:09 AM

76 VETTE

Melting Slicks

Joined: Oct 2005

Posts: 2,114

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From: upstate N.Y.

I went from 3:08s to 3:70s on my 76.It was quite a difference. On the highway seemed like engine was working too hard and ran hotter.I then put3:55s in it .was the best of both worlds. Nice around town and good on highway.Had to change drive gear in tranny to get speedo right.