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Old 07-18-2008, 02:29 PM   #1 (permalink)
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Worth changing gears to imporve Fuel Economy?



So, my question is, Would changing my Mustang's stock 3.27 rear gears to 2.73 increase my fuel economy and by how much possible? I have the scangauge II now and am getting between 24-25MPG this tank so far with 2Gals used. how much MPG increase would this be? Also could I change the driveshaft to Aluminimum to increase FE or would that just hurt FE because it would be lighter and lose momentum fast while coasting? Also I was told that my rear is 7.5" is it possible to increase that to an 8.5" to acheive better FE or does that defeat the purpose as well? I also have a question, which I don't know if this is the same thing but how can I improve acceleration without using 2GPH at 1365RPM when staring off at a dead stop without hitting the accelrator hard at all. Because that is what is really killing my FE from breaking to consistent 26MPG. With the scangaugeII is it possible to limit the amount of gas the throttle intakes at a certain RPM speed. And I know some engines change gears under load, but my Mustang only changes at Certain RPMs say from 0-15 1st gear, then at 1250RPM(21-22MPH) it will switch to 2nd and be at 1000RPM using slightly less fuel. then once it reaches 30MPH at 1500RPM it will switch to third and drop to 1250RPM. Then at 40MPH at 1550RPM it switches to Fourth gear and I'm not sure if 4th gear s OD or not cause it said it's a 4Speed Auto W/OD but around 47MPH I can get 45MPG and on the highway ScangaugeII reprots 33-35MPG at 55MPH. Is there a way to increase FE somehow knowing this like with a tuner or something?

You don't have to answer them all, Even If you know only one answer to this it will be greatly appreciated Thanks

EDIT: I do have a CAI I was looking at threads based on WAI, stock or CAI shouls I keep what I have go back to stock or ge a WAI?????

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Old 07-18-2008, 02:32 PM   #2 (permalink)
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I would say to go for the gear swap. It should be simple on your mustang, right? The other questions are a bit more complicated though...perhaps when I wake up,
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Old 07-18-2008, 02:57 PM   #3 (permalink)
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I worked on Mustang FE in a former life. The axle ratio swap will definitely give improved mileage, though somewhere on the order of perhaps half a mpg. May or may not be worth the investment but certainly if you can get it for cheap and DIY

Aluminum driveshaft will give better FE from weight and reduced inertia - drilling out a flywheel will have the same effect. I cannot think of any real world situation where an Al driveshaft will hurt.

IIRC the 7.5 and 8.5 refer to the diff gear size, in which case I'd recommend you stick with the smaller (lighter) one. The bigger one can handle more torque if you're doing burnouts and the like which I assume you're not. Besides being lighter, less fluidic losses.

Don't have an answer to your last questions. The "K" factor of your torque converter is at fault here but I don't think you can change that easily.
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Old 07-18-2008, 06:42 PM   #4 (permalink)
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The final drive ratio swap will improve highway FE, absolutely. Maybe a mile per gallon, likely more. Its effect on city driving, below 45 mph, is a LOT iffier. I wouldn't count on any gain there (you'd likely delay upshifts, that's about all) so the average engine speed can remain about the same. But if you do highway driving the most, you'd be seeing a real gain.

Of course, if you could recalibrate the shifts to work at the same vehicle speeds, then you'd be more likely to gain in city driving, too. I'm not sure how it's now calibrated -- to work off real vehicle speed or transmission output shaft (driveshaft) speed. If it's the latter than you'd definitely just delay upshifts with a lower numerical final drive.

Your four speed auto has an overdrive (ratio less than 1:1) in top gear. The somewhat higher FE you're seeing after being in 4th already, at a higher vehicle speed, is from the torque converter clutch locking up. That is DEFINITELY a good thing for FE.
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Old 07-18-2008, 11:49 PM   #5 (permalink)
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New gears could be as easy as just swapping in a new rear end. IIRC 00 Mustangs still used a solid rear axle so it shouldn't be too hard. Will net you gains highway cruising.

Lowering rotational mass always helps.

The aftermarket on Mustangs is huge, there should be some kind of reprogrammer you could get for your ECU To change the shift points.
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Old 07-19-2008, 01:11 AM   #6 (permalink)
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Swapping to an 8.8" (the GT rearend) will do nothing for FE, You're way better off sticking with your 7.5" and putting in the 2.73's. This may or may not help though depending on where your power curve is. I would suggest talking to other v6 mustang owners about that before you do the swap. I'm a member at moddedmustangs.com and there are some pretty knowledgable people over there in the V6 section who can probably answer that question. If you do go over there to the V6 section, be patient with your question though, there are a lot of younger kids who like to hear themselves talk in there so you have to sift through their answers.

Red you are correct. All mustangs except the 03-04 Cobras are solid axle rear ends.

I wouldn't tackle a gear swap if this is your first time, personally... Unless you are very confident in your mechanical skills. You're way better off having a shop do the work for you. Also do the math first. Get an idea of how much MPG you can gain by the swap, and then, with the current gas prices, see how long it will take you to recoup the money spent on the swap. It may or may not be worth it, especially since you already have 3.08's. There's not a huge difference between the two. I would also do the same with the driveshaft, you probably won't see much of a gain with it, and they cost a pretty penny.
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Old 07-19-2008, 05:31 AM   #7 (permalink)
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Im not positive but I think maybe my old fairmont project car has an even taller 7.5 gear than a 2.73. Like 2.47? I dont know. It was a 6cyl auto car. And they werent no quarter milers lol.
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Old 07-19-2008, 03:46 PM   #8 (permalink)
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2.73's are too high for that set up. The best bet is to go to 3.08's from the 3.27's. The computer and transmission are already set up for the 3.27's.
You go to 2.73's you will take your car way out of it's power band and confuse the transmission of where it should be shifting. Don't you love computerized cars we have today. In most cases, you noticed the word MOST the automobile manufactures drop the optimum gearing by a set to improve the acceleration. It is the happy medium between fuel mileage and drive ability that is built into the cars today. It's like tire pressure.
They recommend 34 psi for the ride although 40 psi would be much better for fuel economy. There is always a compromise.
Around town there will be a minimum improvement in fuel mileage but expect between 1 and 2 mpg better on the highway. The trick is to incorporate multiple improvements to maximise the fuel mileage gains.
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Old 07-19-2008, 07:41 PM   #9 (permalink)
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Having done the gear swap thing (3.73 to 3.08) I can tell you it is effective. I gained 3 MPG from doing it. But I have a diesel known for its huge low-end torque.

Before you do a gear swap, I'd recommend a long talk with an automatic transmission expert. A numerically lower rear end will slow down your torque converter and reduce ATF flow through the cooler, attenuating cooling. In a pickup, this leads to spectacular transmission failures - parts and fluid scattered down the road. Been there, done that.
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Old 07-19-2008, 08:07 PM   #10 (permalink)
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ALS and garys_1k -

Quote:
Originally Posted by ALS View Post
2.73's are too high for that set up. The best bet is to go to 3.08's from the 3.27's. The computer and transmission are already set up for the 3.27's.
You go to 2.73's you will take your car way out of it's power band and confuse the transmission of where it should be shifting. Don't you love computerized cars we have today. In most cases, you noticed the word MOST the automobile manufactures drop the optimum gearing by a set to improve the acceleration. It is the happy medium between fuel mileage and drive ability that is built into the cars today. It's like tire pressure.
They recommend 34 psi for the ride although 40 psi would be much better for fuel economy. There is always a compromise.
Around town there will be a minimum improvement in fuel mileage but expect between 1 and 2 mpg better on the highway. The trick is to incorporate multiple improvements to maximise the fuel mileage gains.
Without knowing the nuts and bolts of it, that's what I was afraid of. Unless you can also override the ECU/PCM with some kind of "manu-matic" paddle-shifter arrangement, I would think you'd have lots of shifting problems.

Quote:
garys_1k writes :
...

Of course, if you could recalibrate the shifts to work at the same vehicle speeds, then you'd be more likely to gain in city driving, too. I'm not sure how it's now calibrated -- to work off real vehicle speed or transmission output shaft (driveshaft) speed. If it's the latter than you'd definitely just delay upshifts with a lower numerical final drive.

...
Can this be done? At least for a Mustang, I would think there is more aftermarket options for something like this.

CarloSW2

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