2015 GT Gearing upgrade, yes or no? - Mustang Evolution

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Old 04-01-2019, 02:44 PM   #1
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2015 GT Gearing upgrade, yes or no?

Ok guy's I'm getting ready to wake up my pony and I need some advice from those that have been here before me. I have bone stock 2015 GT, adding a stage 2 power kit to it in the next month, which consists of cold air intake, long tube headers and cat-back exhaust, programmer and a short throw shifter; I was also looking at swapping the 3:31 gears out for 3:73, but not sure I'd gain that much from this change; and by this time next year I should be sporting a supercharger on it as well. Would you just leave the lower gearing in or bump it up? It's not a daily driver, but I do like to get out and drive on the weekends and have been averaging just over 25 mpg on the highway, would I kill the mpg if I go to the 3:73's? And by how much? Will the smile on my face make up for any MPG loss if I go to the quicker gear set?
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Old 04-01-2019, 03:03 PM   #2
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3:31 is easier to street on your manual. 3:73 may get under your skin on busy street. mpg my drop to 19-20 on highway 15-16 in city. But you do get more solid accelarations
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Old 04-01-2019, 03:28 PM   #3
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Ahhh.....an area that I can add a lot of value as I have run 3.31's, 3.55's and 3.73's all in the same car before S/C and after.

3.31 gears....they suck with a manual transmission. You need to change them. Until you do, you will never truly love your Mustang.

3.73 gears....they are a ton of fun....for a while. With the mods you are planning initially, you will likely love the 3.73's when you are screwing off. Howerver, you won't like them when you are driving on the highway as they will put the engine in a RPM that will drone at highway speeds. You're mileage won't be ruined...remember that you still have 6th gear at 65MPH.

Once you put the S/C on...you will rue the day you installed the 3.73's. 1st gear will go by so quickly it will effectively be useless. Now...if you intend on tracking the car (road course), then the 3.73's with a Supercharger will be great as the combination in 3rd gear will be killer on the track. But, very few people track their car. For most Mustang owners, they will spend their life with their car running through the gears and an occasional drag race on the street or at a dragway. In that case you will hate the 3.73's because you will hit the rev-limiter in 4th gear before you hit the trap at the drags and you will go through first gear so fast after installing that S/C that you will learn to hate those gears.

CONCLUSION
Put in a set of 3.55's!!! They will lower the gearing enough to be spirited with the initial mods you are planning. You will notice a massive difference in the car in terms of launch and it won't drone at highway speeds. And...when you put on that S/C you will have a big smile knowing that you took the right path at the get go!

I have a 2015 with countless mods (see Building a Super Snake Clone posts), and have settled on 3.55 gears with a 2.9L Whipple S/C. Eventually (next year) it will have Methanol injection. I love the 3.55's. I have an authentic Super Snake (2.9L Whipple S/C with 3.73's)....it is great at a road course but I really dislike the 3.73 gears while driving around on the street and I end up topping out speed-wise in 4th gear about 200 feet before the trap at the drags...which is very frustrating.
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Old 04-02-2019, 06:20 AM   #4
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Thanks so much for info, exactly what I was looking for. I've been following your Super Snake build since joining last year, and I'm planning on following your lead to build up mine as well.
Again thanks for the info!
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Old 04-02-2019, 09:08 AM   #5
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My 14 came with 3.31's & I hated it, the 3.73's in it now make for a more enjoyable driving experience. ( 1st gear is 2.66 )
My 11 came with 3.55's, & I never considered changing them. ( 1st gear was 2.98 )..
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Old 04-02-2019, 01:50 PM   #6
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When you get the differential, it likely will have no gear oil. Use the recommended ford fluid (75W85) and be sure to put in the friction inhibitor/modifier. Sometimes the goofball at the factory parts counter won't give you the friction inhibitor. Get and and put in the 4 ounces or so that are required.

Run the car for a few hundred miles and change out the fluid. The stuff isn't cheap, but this will help give your rearend a good start in life.

Don't be afraid to change that fluid often. I think the suggested change is 30,000 miles or so. If you run the car hard, change it much more frequently. I plan on putting about 5,000 miles a year and will change it annually since many of those miles each year will be hard. If I run a lot of track events, I will likely change it even more frequently.
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Old 04-02-2019, 04:59 PM   #7
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If you are eventually going to go forced induction, don’t go any deeper than the 3.55’s. Otherwise you’ll be so traction limited that you won’t be able to launch the car hard at all......
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Old 04-03-2019, 03:07 PM   #8
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If FI is in your future keep the 3.31's. Many that have 3.73's look to drop back to 3.31's when going FI.
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Old 04-08-2019, 08:02 AM   #9
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I have a 2016 GT PP with the Roush RS3 conversion. Since the PP came with the 3.73 it is the way it is... I agree that it is just too tall, I start out in second sometimes just because when I hit the gas in 1st it is not physically possible to shift fast enough before bouncing of the Rev Limiter. I have considered going to the 3.55 or the 3.31.



MPG is not that bad, drove from Chicago to Flint, MI, and averaged around 23 mpg. My Average MPG is right around 20-21, but I live in the country and I have a mile between stop signs. My trip to work is 4 lights, 15 miles of highway and 4 miles of non highway.
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Old 04-09-2019, 11:05 AM   #10
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Track results with modified 2015 Mustang GT, Getrag M82 6 speed, 3.55 gears, Whipple 2.9L supercharger.

I ran the clone at New Hampshire Motor Speedway "driver's day" this past Saturday. So, these are road course results, not dragway results. The 3.55's performed very well in a small autocross course setup in the parking lot. Was able to get up to speed without needing to shift out of 2nd (it was a very small course) but was also able to have plenty of torque in tight corners without having to shift into 1st. If I had 3.33's there is no way I could have run the car without constantly shifting between 1st and 2nd.

With that said, I can see why Shelby puts the 3.73's in the Supersnake. It is a dedicated roadcourse car and the 3.73's would have performed better than the 3.55's on this course. But, the 3.55's worked fine. And, since I want normal street performance as well as dragway performance...combined with road course performance, I can conclusively now say that the 3.55's are the way to go with a S/C car in which you want to do it all.
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