3.15 vs. 3.55 rear gears in a '16 GT w/Auto trans??

[VooDooDaddy]

Hello fellas,

I've only had my 2016 GT since early November, and I've only driven it about 1,000 miles.

Most of that time I was just driving around with the transmission in DRIVE and the car was just sluggish as hell, and I was considering changing out the rear pumpkin from the factory installed 3.15 ring/pinion to a 3.55 pumpkin to give the car some pep.

Then I started driving it more and more in SPORT and SPORT+ mode. This made the car much more lively and fun to drive, and now I'm thinking a gear swap may not be necessary??

When in SPORT mode, first gear is pretty much useless, and when I paddle shift the car, I am out of first gear almost immediately once the car is moving.

As well, I just picked up a SCT X4 tuner from another forum member, and will be going with a LUND 93 tune within the next month. From what I have read from some of you nutters here, this impending LUND tune will definitely make my car a bit more spirited? I also replaced the stock air filter with a K&N, installed a UPR catch can, and I have a set of MAK cat deletes going on the car this weekend.

My 2016 GT is not my Daily (Chevy Cruze for that), I have no plans to race or track the car. I just want a nice toy to play around with and have some fun taking the wife out every now and then.

Would a gear swap be worth the effort (I'll be doing the labor) and expense, at about $500 for a used 3.55 pumpkin?

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[DJinAC]

My 15 GT auto had the 3.15 rear end. I wished it had the 3.55. Even the 3.31 would have been better than the 3.15 gears. My 15 GT with the manual had the 3.55 and it was a huge difference, to me anyway.

 

[gearhead2685]

I have a 16 A6 black accent car with the 3.55s and I love it. Im almost considering putting 3.73s in the rear once the CJ Intake Manifold is on.

Unless you are after mileage or do alot of highway/interstate driving I wouldnt want the 3.15s

The 3.15s seem to work best for boosted cars so they dont blow the tires off.

 

[gameovergt]

3.55's from factory & love them!

 

[VooDooDaddy]

Thanks for the replies fellas, looks like I am on the hunt for a 3.55 pumpkin!

 

[Dr. Norts]

3.55's from factory & love them!

Same. 3:55s plus a tune and the car feels awesome.

 

[Coyote Red]

I specifically noticed my dealership salesman, 3.55:! or no sale! I have it and "paddle" the Hell out of it! If you go from 3.15:1 to 3.73:1 you'd need an extra tool to switch, I don't know about 3.15:1 to 3.55:1 if you'd need the extra "tool" to install. Keep us posted on the switch, please.

 

[Dr. Norts]

IIRC the 3:15 pumpkin can't fit any higher gears than the 3:15. The pumpkin that comes with the 3:55 is needed.

I could be getting confused, so someone correct me if thatss incorrect please.

 

[VooDooDaddy]

IIRC the 3:15 pumpkin can't fit any higher gears than the 3:15. The pumpkin that comes with the 3:55 is needed.

I could be getting confused, so someone correct me if thatss incorrect please.

You are correct, and as I've said above I will be replacing the entire pumpkin/differential and not just the gears.

As well, the cost of just replacing the gears (if it were possible, its not) you would need the ring/pinion, and shim and crush sleeve kit, and have to pay someone to properly set the lash/backspacing.

In the end, it will be cheaper for me to replace the entire pumpkin as I can do the swap myself.

 

[GreenS550]

I believe that is incorrect. I believe the pumpkins are the same, only the ring and pinion are different.

Back to the OP. When I bought my 2015 used I looked for the 3:55. But, the tune will be way more important. You won't recognize your car after the tune. Please post your results.

Also, if you go to drag times, look up some 1/4 mile times for the S550 auto with the 3.15 gears. They go pretty fast! Wait for the tune. Then decide on the gears. 3.15 will be a better gear for a roll on race...

 

[GreenS550]

The competent shop can do the gears for 300-400 labor and the gears are under 200, so you are still ahead to have the work done. The automatic transmission needs another part besides the Ford Racing pumpkin.
Wait for the tune.
I remember installing 410 gears in my 2005 auto GT and it completely changed the car. Awesome! But in my SC Bullitt 410s were worse than my 373s because of the add'l horsepower that car had. It really ruined it.
Your 2016 with a tune is going to make over 400 rwhp for sure and you might just like your 315s.

 

[Strokerswild]

IIRC the 3:15 pumpkin can't fit any higher gears than the 3:15. The pumpkin that comes with the 3:55 is needed.

I could be getting confused, so someone correct me if thatss incorrect please.

Correct.

You cannot put lower (numerically higher) gears in a 3.15 pumpkin...

 

[pjk]

I'm running the 3:55 gears in my 2015 auto and it's great in Sport+ mode. In normal mode it seems fine but really comes alive (I'm guessing due to throttle response change and shift points) in S+ mode. It's a good combination in my opinion (auto plus 3:55s).

I do only get ~17mpg in mixed driving. The highest I've ever gotten was ~23 in all hwy driving so there maybe a mpg hit vs. 3.15. I'm lucky 3.55s and Auto came straight from Ford so I don't have experience upgrading from 3.15s.

 

[GreenS550]

Correct.

I am completely wrong on this. I just learned something.

Sorry for the confusion

 

[gmupatriot]

OP, this is not an orange to orange comparison but my 2018 GT with the A10 has 3.55 rear. I am glad I got it this way because the GT I test drove had the 3.15 rear and though very quick, did not have the punch the 3.55 provided. My experience so far has been great considering the A10 shifts quickly and downshifts to get the car in the power band, the 3.55 helps as well. I am driving the car right outside of D.C. and anyone who lives in metropolitan area knows how bad rush hour traffic gets around here. Given the mixed driving I do, I am able to get 20mpg consistently. The car has ~3000 miles on it now and loving every moment of it.

Once you get the 3.55 rear and a tune, you will be hauling some serious ass!

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