3.31 to 3.73 swap out.

clsinclair · Feb 24, 2019

I am new to this forum so please be patient. I have a 2016 Mustang GT. I have already installed a Racer cold air filter and noticed a marked improvement in performance. I am now going to have a 3.73 rear end installed. It currently has a 3.31 gear. My problem is getting someone to install it. I live in Lancaster, SC. I went to the local dealership and first of all they want $1,750 for the installation and then they didn't know if they had to change out the differential for another $1,000 plus. I told them that the 3.31 differential was interchangeable with the 3.73. Car has 19K miles. The local dealership threw up several red flags regarding their knowledge of the Mustang. Do any of you know of any place in Charlotte, NC that could switch out a rear gear? I am only 19 miles out of Charlotte. Sadly enough I could not find a garage in Lancaster that would or could do the work.

90sickfox · Feb 25, 2019

Wish I was down there too point you in the right direction. Places are all over maryland and VA that could do the install. It's not that bad of a job. Really wish I could help you further.

I Bleed Ford Blue · Feb 25, 2019

You could always just buy a loaded center section from ford racing. they are pre setup up from the factory using the same OEM specs. Just bolt it in and add fluid. If you do it yourself, it will save you some coin over what you were quoted.

https://www.americanmuscle.com/ford-performance-irs-loaded-differential-housing-355-1519-gt-v6.html

And they have to drop the center section to regear anyway, so why not just buy a preloaded one and swap it in, save time and money.

Boone · Feb 26, 2019

Is your car a stick car or an automatic?

clsinclair · Feb 26, 2019

Boone said:
Is your car a stick car or an automatic?

It is a stick.

Boone · Feb 28, 2019

clsinclair said:
It is a stick.

Go for the 3.73's, if it were an automatic I might not. 3.73's will go on your diff, but won't go on a 3.13 diff.
3.73 comes in the PP cars and its not too much gear for a MT-82 car for sure.

clsinclair · Feb 28, 2019

Boone said:
Go for the 3.73's, if it were an automatic I might not. 3.73's will go on your diff, but won't go on a 3.13 diff.
3.73 comes in the PP cars and its not too much gear for a MT-82 car for sure.

I am having the 3.73 installed. Took the car in on Tuesday and so far the incorrect parts were ordered plus a flange that was not needed. You would think better for a dealership. Another place was going to charge a few hundred dollars lower but they were going to take it to another town and said that it would take a week. I could switch out a hog head (differential) during the 60's and 70's and then things got a little more involved and I didn't keep up with the changes. Wish now that I had. Fact is my brother and I could lay out a 327 part by part on the driveway and put it back together.

Boone · Mar 1, 2019

Things have changed over the years but the basics are the same. If you've been turning wrenches all your life, you will catch up in no time. The electronics are probably the biggest departure, but they are amazing!

clsinclair · Mar 1, 2019

Picked up the car today and everything seems good. They could not re-calibrate the speedometer. They said that they would have to reset to factory defaults so since I have a Bama tuner I will do it myself. As far as I can tell the speed is only off about 2 mph when matched with my GPS. I may just let it be but with my OCD I will have to do it myself anyway. At 60 MPH in 6th gear I was turning about 1,700 rpm and averaged about 27 mpg coming home on the highway. We will see how all of it works out. Since it was raining I didn't give it much gas.

Boone · Mar 1, 2019

What size tires are on the back? Had you checked it before you dropped it off?

clsinclair · Mar 1, 2019

Boone said:
What size tires are on the back? Had you checked it before you dropped it off?

235/50/18

Boone · Mar 1, 2019

That's 27.3" tall, sounds about right. Is that the size tire the sticker on the door jamb calls for? I haven't been inside a super 8.8 but I'm curious because the tone exciter ring should be on the carrier and changing the gear set shouldn't affect the speedometer. The speedometer uses the same exciter ring as the anti-loc brakes use if I recall correctly.

clsinclair · Mar 2, 2019

Boone said:
That's 27.3" tall, sounds about right. Is that the size tire the sticker on the door jamb calls for? I haven't been inside a super 8.8 but I'm curious because the tone exciter ring should be on the carrier and changing the gear set shouldn't affect the speedometer. The speedometer uses the same exciter ring as the anti-loc brakes use if I recall correctly.

I will check later today. Before I took the car to the dealership the speed was off about 2 mph the other way and now it is reversed with the 3.73. I checked it with a separate Garmin GPS. I can live with the 2 mph difference but it gives me a great opportunity to play with my tuner. This will most likely not do any more mods but who knows?

Colorado66 · Mar 2, 2019

clsinclair said:
Picked up the car today and everything seems good. They could not re-calibrate the speedometer. They said that they would have to reset to factory defaults so since I have a Bama tuner I will do it myself. As far as I can tell the speed is only off about 2 mph when matched with my GPS. I may just let it be but with my OCD I will have to do it myself anyway. At 60 MPH in 6th gear I was turning about 1,700 rpm and averaged about 27 mpg coming home on the highway. We will see how all of it works out. Since it was raining I didn't give it much gas.

Not sure which Bama tuner you have, but for my '14 V6, I have the X4/SF4, purchased from AM which includes 'Free tunes for life'. This allows user selection for some criteria, such as top speed limiter, tire size (in 'revolutions per mile') and final gearing, etc.. From my experience, with this particular tuner, the last tuner files provided (by AM) were incorrect (255/40R19 tires' diameter of 27.0" calculated to about 746 RPM and my tuner file was set at ~790, resulting in speedo indicating about 11MPH lower than actual); despite the advertised ability for resetting this by the user, apparently the user selections (745 RPM) would NOT override the tuner file and I had to go back to AM to have a new file sent with the correct tire RPMs. I just installed the new tune files yesterday and have not driven it since, so I'm not yet sure that this has resolved the issue. Bottom line of the cautionary tale, if you're not able to correct the speedo error w/ your tuner by adjusting the user-defined criteria, you may have to go back and request an updated file (assuming that this service is available to you by your seller).

Also (and likely too late), I too have parts on hand for a 3.13 > 3.73 gear sway for mine. While it was a few years back, the gear change on my '66 at my local transmission shop ran about $450 for labor only (plus parts, which I ordered/provided). While I know it varies from one dealer to another, some just do NOT like to deal with installation of non-stock (and particularly non-FoMoCo aftermarket parts). As an example, my local Ford dealer has turned me away, refusing to install items such as a non-Ford rear deck lid spoiler (neither they nor I wanted to drill through the trunk) and Eibach progressive-rate lowering springs (I was told that they "didn't have the right spring compressor" ?!?). While other Ford dealers may be more apt to work on non-OEM parts' installs, mine seem less willing to do so. I don't have any heartache with this, as generally speaking, their shop rate is MUCH higher that other competent alternatives.

clsinclair · Mar 2, 2019

I have the Bama Rev-X tuner. The original tune was for 93 octane fuel and a C&L racer cold air filter. Since I just changed from a 3.31 gear to a 3.73 gear will have need to get an update from American Muscle or will it already be a choice on the tuner?

Colorado66 · Mar 3, 2019

clsinclair said:
I have the Bama Rev-X tuner. The original tune was for 93 octane fuel and a C&L racer cold air filter. Since I just changed from a 3.31 gear to a 3.73 gear will have need to get an update from American Muscle or will it already be a choice on the tuner?

Assuming that the software is the same in our Bama tuners, the rear end gearing is one of the criteria that you SHOULD be able to change yourself during the process of uploading the tune to your vehicle; just select the proper gearing ratio (3.73 ) from the options list when prompted to do so. As I was told, the new tune will not necessarily 'take hold' until you've driven about 50 miles, IF, at that point, you find that your speedo is still off, you then will need to contact AM Tuning to request that this be re-set (within the tune file) and then they can send you an updated tune file(s).

clsinclair · Mar 3, 2019

Colorado66 said:
Assuming that the software is the same in our Bama tuners, the rear end gearing is one of the criteria that you SHOULD be able to change yourself during the process of uploading the tune to your vehicle; just select the proper gearing ratio (3.73 ) from the options list when prompted to do so. As I was told, the new tune will not necessarily 'take hold' until you've driven about 50 miles, IF, at that point, you find that your speedo is still off, you then will need to contact AM Tuning to request that this be re-set (within the tune file) and then they can send you an updated tune file(s).

I purchased the tuner during October to tune my 2016 GT for a cold air filter and 93 octane fuel. I downloaded that tune to the car during the same time. So what would I need to do to download the 3.73 rear axle information?

Colorado66 · Mar 3, 2019

clsinclair said:
I purchased the tuner during October to tune my 2016 GT for a cold air filter and 93 octane fuel. I downloaded that tune to the car during the same time. So what would I need to do to download the 3.73 rear axle information?

Assuming no other changes, and again assuming that the software in our tuner is the same...
(1) Start your Device Updater software on your PC; plug your handheld tuner into the PC and run the [check for new software] function; update software if applicable and then unplug your device from the PC
(2) Plug your handheld device into your car's port and then run the [progam vehicle] function, just as you did initally when you installed the original tune... Follow the prompts as directed; when you come to the page allowing you to check the user defined features, select 3.73 for the gearing ratio and then proceeed through the remainder of the loading process.

If you tuner's software is different than mine, I can't be of any futher help; at whcih point you'd need to call AM Tuning at (888) 226-9764 for instructions.

Hope this helps,
pws

clsinclair · Mar 3, 2019

Colorado66 said:
Assuming no other changes, and again assuming that the software in our tuner is the same...
(1) Start your Device Updater software on your PC; plug your handheld tuner into the PC and run the [check for new software] function; update software if applicable and then unplug your device from the PC
(2) Plug your handheld device into your car's port and then run the [progam vehicle] function, just as you did initally when you installed the original tune... Follow the prompts as directed; when you come to the page allowing you to check the user defined features, select 3.73 for the gearing ratio and then proceeed through the remainder of the loading process.

If you tuner's software is different than mine, I can't be of any futher help; at whcih point you'd need to call AM Tuning at (888) 226-9764 for instructions.

Hope this helps,
pws

I followed your instructions and it seemed to work. I had to manually key in 3.73 on the tuner. I took it for a spin and the difference in the speedometer was less than 1 mph. I can live with that. Thank you very much and thank all of you for your input.

Colorado66 · Mar 4, 2019

clsinclair said:
I followed your instructions and it seemed to work. I had to manually key in 3.73 on the tuner. I took it for a spin and the difference in the speedometer was less than 1 mph. I can live with that. Thank you very much and thank all of you for your input.

Great (and de nada)!

3.31 to 3.73 swap out.

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Wasn't a pretty sight...and I've got big hands

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