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Old 09-17-2005, 06:54 PM
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proper rear diff ratio

Well, I am plodding away selecting parts for my 52 chevy sedan project, and I am trying to figure out the proper gear ratio. This is entended to be a nice driver/cruiser with good performance and economy in mind. The engine is going to be a mild 350 stroker and the trans is going to be a 2004R. Knowing this, most all advise I am getting is to use a 3.42 rear end which makes sense as that is what is in my 87 Regal T. While cruising the GN site I found that the 2004R was also used behind the 5.0 V8 and the 3.08 rear was used. My question is why would one rear be used in favor of the other, especially when the transmissions are the same. Is it the choice of engines in this case? If so would the 3.08 be an OK choice? Thanks.

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Old 09-17-2005, 07:20 PM
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308 is a good highway gear like interstate roads, 373 is good hp gear and curise gear, 342 also
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Old 09-17-2005, 08:59 PM
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I think the 3:08 will be too low. At 70mph in OD your rpm is going to be really low, probably in the 1000 to 1500 range. a 3:55 or 3:70 would be better imo. A 3:70 with a .7 OD and 29" tires is about 2100rpm at 70mph. There are various rpm calculators on the net to figure what rpm you are shooting for at what speed and with what gear ratio.

Vince
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Old 09-17-2005, 09:43 PM
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In addition to interstate driving, I think you need to consider other driving. I drive frequently at 40-50 mph on suburban four lanes, and with the OD transmission I am at low rpms in steady state speeds in this range.

I have the TH200 4R with 3.42 gears and 27 inch tires. At 40 mph in OD and the converter locked up, I am only turning 1200 (calculations say it should be 1100 rpms but the tire compresses some and only has an effective diameter of 25.5 inches). With a 3.08 I would be turning 120 rpm less.

So give some thought to your overall driving patterns, not just interstate speeds, unless that is essentially all you drive.

I am considering putting in a TCI lockup controller that prevents torque converter lockup unless the car is going more than 45 mph. That will add about 200 rpm to the engine speed at speeds below 45 mph.

John
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Old 09-18-2005, 12:29 AM
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You need to know what size tires you are gonna be running before you start talking gears.
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Old 09-18-2005, 12:42 AM
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Quote:
Originally Posted by 302/Z28
I think the 3:08 will be too low. At 70mph in OD your rpm is going to be really low, probably in the 1000 to 1500 range. a 3:55 or 3:70 would be better

Vince


Vince is correct......

The transmission has a .33 overdrive.

A 3.73 minus 33 per cent is 2.49 ( in overdrive)
A 3.42 minus 33 per cent is 2.29
a 3.08 minus 33 per cent is 2.06

WAY to low IMHO....

Most camshaft companies list cruising RPM.....for a reason. You can hurt mileage and performance by gearing a vehicle too much ( it lugs the engine ......too slow... )

A 4.10 minus 33 percent is 2.74
Great acceleration and a good cruise RPM too...
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Old 09-19-2005, 07:49 AM
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You will need to figure the most of what type of driving you will be doing, then figure in a few variables from there to get the whole thing right for that particular driving type.

You will most likely want a 50-55mph cruising range of 2200 rpm's (this range gives a little leeway and you can choose within this range).

Your tire size will affect everything else and should be chosen AFTER the rest.

Your gear ratio is limited to pretty much 4 different gear sets and each can be offset either way by the tire size.

Your transmission is the only part that is not a variable.

I went about trying to get better that the 13 mpg average in my 1981 Malibu,
I knew the TH-700R4 OD ratio was .70, and I was not building the engine RPM's to where the power band of the camshaft was making the best power with the best mileage as an average.
I found out the original gears in the car were 2.29's and this was more than enough gear reduction for cruising the long straight stretch highways with NO hills to worry about, but I don't live in any part of the country like that.
I set my cruise RPM goal at 2200 for my particular engine at a vehicle speed of 55 MPH.
Online calculators gave me indication my goal would be best acheived by using a tire size of 26 inches in diameter while using a 3.73 ratio.

I have plenty of power, downhill engine braking, 55 MPH cruise around 2200 with the torque converter locked up, and am using a set of 235/60/15's.

I am now getting an average of 19 miles per gallon figured at each fill-up.
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Old 09-20-2005, 12:06 AM
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Quote:
Originally Posted by M&M CUSTOM
Your tire size will affect everything else and should be chosen AFTER the rest.

No, you determine what tires you are going to use, THEN select your rear gears to get the proper ratio.
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Old 09-20-2005, 08:57 AM
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Ditto what Bruce Wang said.

Decide what tires you will be using to give the right fit and look you want. Then, knowing the tire diameter and trans, you can pick a rear gear ratio that will give you the rpms you want to fit your driving habits, i.e., what rpms do want to turn at urban commuter speed, country cruising, or interstate driving -- and you will have to compromise somewhere. An OD trans helps a lot.
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Old 09-20-2005, 10:01 AM
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Thanks for all the replies. I am going to fiqure out the tire size and go from there. I do have a rear diff that came out of a later model camaro that is a posi trac (10 bolt) and is the right dimensions for the car. It is a 3.08. I may just try that and if I need to I can change the gear set later.
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