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After I get new wheels and tires I was considering my next modification to be rear end gears. Have any of you replaced or bought an automatic GT with 3.55 gears? If so, what is your opinion of them? Was it worth the swap? I read online that some auto's can come with 3.55's from the factory but I don't know for sure if that is true. I have the stock 3.15's at the moment and was considering changing them. I do not want to kill my highway gas mileage so I was leaning towards 3.55's vs 3.73's or the 4.10's.
 

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I think most people who ordered their cars went with the 3.55 gears.They seem to be a good compromise in my car. The mileage cant be much,if any different.
 

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Automatic GT's are faster in your typical drag race with stock 3.15 gears.

With that out of the way, what is your main goal with the car?

If you want to drag race in the quarter mile, then your gear selection should be match/selected with the power and grip you have. For a stock car, the 3.15's work really well.

If you're just wanting a little more response from each gear, 3.55's would be a good balance while retaining fuel mileage, though 4.10's would be very similar to 3.73 in a manual GT except for on the highway. The Auto trans has a lower 6th gear ratio which translates to more RPM's.
 

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For most with an automatic, the biggest gains seem to have come from a new torque converter vs a gear change.

That was the case with the S197s and it probably isn't going to be any different with the S550.
 

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For most with an automatic, the biggest gains seem to have come from a new torque converter vs a gear change.

That was the case with the S197s and it probably isn't going to be any different with the S550.
This is true, but only if the OP is looking to hit the drag strip, otherwise overkill for a daily driver.
 

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I think that you cannot install 3.31 or higher ratio gears in a 3:15 housing. Don't know why but the Ford Racing book says they won't fit. Does anyone know what would have to be changed to install 3:55 gears in my 3:15 2015 GT? Do I have to change the whole pumpkin or can the carrier be swapped?

Update: Looks like the gears and the posi have to be swapped if you're going from 3.15 to any higher ratio, 3.31 and up are ok.
 

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You need to decide on your tire size first. also mine has 3:31s not 3:15? If you go to a 20" tire it may throw your final drive ratio off. You can order a 3:73 from the factory, as far as I know there is only one size pumpkin! 3:73 gears have been more or less a standard for decades, your stock gears are 3:31s not 3:15s. With bigger tires the 3:73s will actually give you better mileage in town, but how and where you drive has way more to do with gas mileage than gears.:wink:
 

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The post above is correct for a manual tranny not the auto. The auto stock is 3.15 and optional is 3.55 from the factory.

I'm contemplating getting a 3.55 gear car from the factory but if the 3.15 is still the better option in a typical drag race I'll stick with that. I'll probably just add ford racing parts that are similar to the track pack as I go instead to gain performance since a track pack can't be done with the auto unless it's a 50th anniversary edition from what I've read.
 
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