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Old 04-06-2013, 04:10 AM   #1
Jacobpockros
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Default What are my Final Drive options?

Just bought a 2001 Legacy GT Limited Sedan 5-Speed. I love the car but need a lower final drive for gas mileage.

From searching the forum, I found that it uses a 4:11.

The base model 5-Speed of the same year uses a 3:9. Is this the lowest ratio rear diff that can be swapped in or is there anything else with a lower ratio than 3:9 that's bolt in?

Also, would I need only a Rear Diff or both Rear and Front?

Lastly, does lowering the Final Drive cause ANY incorrect Speedometer or odometer readings?


Please get back to me guys.

Thanks,
- Jake
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Old 04-06-2013, 08:21 AM   #2
Duvanator
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You would need both front and rear. And the front diff per say is in the transmission, so it would involve opening and swapping the required cogs/parts.
If you were to do just the rear say goodbye to the center diff lol
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Old 04-06-2013, 08:24 AM   #3
Charlie-III
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1-You need to do both diffs, front & rear, if you want to last more than a few miles on pavement.
2-Yes, you will have a speedo/odometer error unless you change parts.

Cheaper/easier would be to run a taller tire than stock. What kind of mileage do you get now?
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Old 04-06-2013, 01:53 PM   #4
geronimo81
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Unfortunately your car won't ever get "good" mileage. However if you want to keep the car then yes you might want to switch to the 3.9 final drive. As mentioned above, you will need both the front and rear diff.

I like the taller tire option except you are going to be limited in how tall you can go while raising your profile will cause greater wind resistance.
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Old 04-06-2013, 05:58 PM   #5
silverton
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honestly, I'm impressed my '01RS gets 23-24mpg, being a 2.5L 4 cylinder with a 4.11 and all wheel drive.
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Old 04-06-2013, 10:00 PM   #6
Jacobpockros
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Okay. Thanks guys
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Old 04-07-2013, 12:46 AM   #7
geronimo81
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Quote:
Originally Posted by silverton View Post
honestly, I'm impressed my '01RS gets 23-24mpg, being a 2.5L 4 cylinder with a 4.11 and all wheel drive.
That's highway mileage? I used to get 28-29 on the highway with 17's on my RS
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Old 04-07-2013, 01:16 AM   #8
Jacobpockros
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Quote:
Originally Posted by geronimo81 View Post
That's highway mileage? I used to get 28-29 on the highway with 17's on my RS
Sounds predictable.
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Old 04-07-2013, 02:38 PM   #9
Grindbeard
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Shame you live in CA, I'd be more than happy to swap my 3.90 for your 4.11!
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Old 04-07-2013, 03:48 PM   #10
silverton
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Quote:
Originally Posted by geronimo81 View Post
That's highway mileage? I used to get 28-29 on the highway with 17's on my RS
Mixed. it gets 29.5'ish when it's all freeway.
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Old 04-09-2013, 07:57 AM   #11
Charlie-III
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Quote:
Originally Posted by Jacobpockros View Post
Okay. Thanks guys
What are you getting for mileage??

I have a 98 GT wagon and usually get 22MPG mixed driving. Highway driving is almost always ~80MPH.

I had ~27MPG when all highway and back down the speed a bit.
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Old 04-09-2013, 12:38 PM   #12
Patrick Olsen
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Quote:
Originally Posted by Jacobpockros View Post
Just bought a 2001 Legacy GT Limited Sedan 5-Speed. I love the car but need a lower final drive for gas mileage.

From searching the forum, I found that it uses a 4:11 4.11:1.

The base model 5-Speed of the same year uses a 3:9 3.90:1. Is this the lowest ratio rear diff that can be swapped in or is there anything else with a lower ratio than 3:9 3.90:1 that's bolt in?
The '06/07 WRX used a 3.70:1 final drive. The 5MT was changed a bit somewhere in the 2004/05 range, so I don't know if the newer 3.70 gears would fit in your older 5MT. I think they would, but I imagine you'd get better answers in the Transmission & Drivetrain forum.

Quote:
Originally Posted by Jacobpockros View Post
Also, would I need only a Rear Diff or both Rear and Front?

Lastly, does lowering the Final Drive cause ANY incorrect Speedometer or odometer readings?
As others have said, you would need to change both front and rear diff ratios. The easiest way to do this would be to just swap out the complete units (transmission and rear diff). That should mean there wouldn't be any speedo/odo issues. The gear ratios for the non-2.5GT transmission are exactly the same, just the final drive is different. According to cars101.com, the '01 2.5GT had a rear LSD and the L did not, so swapping to the L rear diff would mean that you lose the LSD. Not the end of the world. If you really cared about keeping the rear LSD, then you would need to swap just the ring and pinion onto your existing LSD rather than swapping out the entire rear end.

Another option, rather than changing the final drive ratio, would be to swap out just the 5th gear. The WRX 5MTs have a 0.738:1 5th (compared to your 0.780). That would give you the same drop in highway RPMs (with 10 or 20rpm) as going from 4.11s to 3.90s, yet wouldn't saddle you with the more sluggish performance in 1st thru 4th. I'm pretty sure the WRX 5th gear from the same generation would fit into your tranny, but again there are more knowledgeable folks in the correct forums that would be better to ask.
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