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'69 Fleetwood Brougham, '76 Fleetwood Limousine
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Discussion Starter #1
Has anyone put in a lower (numerically higher) rear axle ratio to improve acceleration?
I'm considering doing that on my '69 Fleetwood Brougham. A change from the stock 2.94:1 ratio to one from a same-year limousine (3.15:1) or even lower (like 3.36:1) would be fun to have. I do realize that fuel mileage would drop and there would be more rpms on the highway but I only drive this car about 3,000 miles/year anyway.

I also plan to install a shift kit.
 

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94 Fleetwood Brougham
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Honestly, take a look where your engine makes peak torque, gear it to be at cruise just below that rpm. That will give great throttle response. Cad's like to cruise low rpm, 3.15 is good, 3.36 might be a little low (numerically high).
 

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'69 Fleetwood Brougham, '76 Fleetwood Limousine
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Discussion Starter #3
Yeah, probably so. I wonder if going from 2.94s to 3.15s would make much of a difference in acceleration?
The peak torque for a '69 472 is at 3,000 rpm (525 ft/lbs). 3.15s should be just about right.
 

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94 Fleetwood Brougham
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yup, you might really like the 3.15's. 455 Pontiacs like 3.08's to 3.23's with stock cams and heavy cars. And they still can run blindling fast (Jim Hand had one running 12.20's with 3.08's and ALL factory parts, just not necessarily the combos the factory did, with 4000# car weight). He swapped in 3.73's, and the ET was the same, and the mph was slower.
 

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Discussion Starter #5
N0DIH said:
yup, you might really like the 3.15's. 455 Pontiacs like 3.08's to 3.23's with stock cams and heavy cars. And they still can run blindling fast (Jim Hand had one running 12.20's with 3.08's and ALL factory parts, just not necessarily the combos the factory did, with 4000# car weight). He swapped in 3.73's, and the ET was the same, and the mph was slower.
Wow... 12.20s with 3.08s is very good!

If I do decide to change gears, it will probably be the 3.15s. And it should be no problem finding that ratio... all the '68-'76 limos used them. And the hearses, I suppose. My '76 Limo has 3.15 gears and it does seem to help it accelerate... very smart of Cadillac to use a lower gear ratio in the limos. Tall gears aren't always the best way to go.

I've seen road tests of two cars of the same type, but one had 2.76 gears and the other 3.23s and the mileage was almost the same because the car with the higher gears lugged too much around town and spent too much time in 2nd gear.
 

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1995 ETC, 75 Deville, Cad500 powered 73 Apollo, 94 Mark VIII
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2.94 to 3.15 doesn't seem like it would make enough differance to do all the work involved. But if you have a lot of spare time on your hands go for it.
 

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1996 Fleetwood
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I made a jump on my 96 Fleetwood, 2.56 to a 3.42 w/ Posi. It matters a ton. The car is pulling a lot stronger off the line, and the posi means she's digging every time. Chirps the 1-2 shift as well. Loving them bigtime, just have to get the speedo fixed and the governor yanked. :)
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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ShadowLvr400 said:
Loving them bigtime, just have to get the speedo fixed and the governor yanked. :)
What do you do to correct the speedo readout?
 

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Discussion Starter #10
davesdeville said:
2.94 to 3.15 doesn't seem like it would make enough differance to do all the work involved. But if you have a lot of spare time on your hands go for it.
Yeah, I'm wondering about that, too. That wouldn't be much of a change. I'll have to think about it and go through the facts and figures... things like the mph/rpm with different gear sets and so on.
I do have some catalogs (like PAW- Performance Automotive Warehouse) which sells Richmond gears, but I don't think they would fit a Cadillac rear axle. Too bad... I would probably go for 3.23s.
 

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Discussion Starter #11
ShadowLvr400 said:
I made a jump on my 96 Fleetwood, 2.56 to a 3.42 w/ Posi. It matters a ton. The car is pulling a lot stronger off the line, and the posi means she's digging every time. Chirps the 1-2 shift as well. Loving them bigtime, just have to get the speedo fixed and the governor yanked. :)
Oh, yeah. That would make a big difference... going from 2.56s to 3.42s. How did it affect the gas mileage? Probably not a big amount.
I went from 2.93s to 3.23s on the '66 Dodge Dart V-8 I used to own. Definitely made a difference. I think for most people, anything under 3.00 is too high (numerically low), unless most of their driving in on the highway.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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ShadowLvr400 said:
In my case, reflash the computer. Sending off to a company that does it. Dunno on your car, might be able to do it through the dealerships.
Thanks. This will be on a diesel 4L80E computer that originally played into 3.73's. That trans is going into my '80 Fleetwood 2 door sometime. I'm not set on what ratio it will be setup with now but i'd like 2.73.
 

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1995 Fleetwood
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I made a jump on my 96 Fleetwood, 2.56 to a 3.42 w/ Posi. It matters a ton. The car is pulling a lot stronger off the line, and the posi means she's digging every time. Chirps the 1-2 shift as well. Loving them bigtime, just have to get the speedo fixed and the governor yanked. :)
I've heard if you did a gear swap that you would have to get the pcm reprogrammed. Is this true?
 

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94 Fleetwood Brougham
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Yup, must program PCM.... Mine was factory 3.42's, loved them from day one. I suspected it when I first drove the car, as you didn't have high mph in first gear.

Best mod!
 
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