Gear Change from: 3.73 (Rear Gear) to: 4.11 in my GT350

Discussion in 'Shelby GT350 Mustang' started by JohnVallo, Jan 10, 2018.

  1. JohnVallo

    JohnVallo 1 5 4 8 3 7 2 6

    First Name:
    2016 GT350, shadow black; chassis: G1245
    Feb 9, 2015
    Venice, FL
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    Retired, (F idelity, B ravery, I ntegrity)
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    #1 JohnVallo, Jan 10, 2018
    Last edited: Jun 25, 2020
    Wanted to share my experience for others interested in working with the newer 'Super 8.8', as well as information on how the project went.
    Been driving my 16’ GT350 around Florida for 2 years now, 8000 + miles on it. About a year in I started wondering about the Final Gear Ratio in the GT350. I am mainly interested in Street Performance now, as a personal preference.
    (I raced Stock Cars on oval tracks up North for 40 years before retiring so I’m done with the track stuff and I just prefer Street Performance at this point.)
    Just so we’re on the same ‘Terminology Page’, here is a chart I made on the different terminology for Most-Common descriptions that people use to describe Rear Gear Ratios.
    3.15 : 1 (Gear Ratio)
    Lower Number
    Higher Gear
    Longer, Taller Gear
    Shallower Gear
    Less Gear

    4.11 : 1 (Gear Ratio)
    Higher Number
    Lower Gear
    Shorter Gear
    Steeper Gear
    More Gear

    Personally, I think because of my Stock Car Racing, I prefer the ‘Less Gear, More Gear’ Jargon.
    Anyway, the more I drove the 3.73 GT350 the more I thought, “This car needs a little more Gear”
    How much more? This is where I began my project. (I was thinking, 4.11 or maybe 4.30)
    SEMA SHOW 2016 -US GEAR- announcement Link:

    Well, the 4.11 or 4.30 question got answered quickly, as I wrote to AxleTech/US Gear and acknowledged that they were indeed making 3.73, 3.91, and 4.11 gears for the 2015-Up Mustang “Super 8.8” IRS rear end, but did they have plans for a 4.30 ratio? The answer was: “NO Plans”. This happened just at the time Ford Racing had advertised a 4.09 gear-set, but suddenly it was not available, so at that time Ford had nothing to offer for the Lower gear choice.
    Well the Ford 4.09 (Ford Performance) dilemma may have had something to do with suppliers, as we then had the 2017 SEMA announcement.
    SEMA SHOW 2017 -US GEAR- announcement Link:

    So, 4.09 (Thru Ford Performance) or 4.11, (US Gear now makes them both), what’s the difference? Well the short answer is: “Tooth Count”, 11 tooth pinion, 45 tooth ring-gear for the 4.09, and 9 tooth pinion, 37 tooth ring-gear for the 4.11. The long answers to my questions, (I never received). My questions were: Which one was stronger? Which one was quieter? Which one was better? Which one would you choose? US Gear did not answer these questions, other than to say: “If your car is still under warranty, you may want to go ‘Ford Performance 4.09’ for warranty concerns.”
    Well I went with: US-GEAR 07-888411-15, Picked one up for $255.00 (including shipping.)
    Also bought: Ford Racing M-4210-B3 (Install Kit) $110.00, as well as: TA-29 (Motorcraft Silicone sealer) $6.00, and 2 quarts of (Motorcraft 75W85 Fully synthetic gear lube) $44.00. Also 2 cans silver high-heat spray paint, $20.00.
    I decided the best thing for me to do was to try and find another rear end from a GT350, rebuild it, so as to keep my original rear end intact. I did luckily find a 2016 Low-Mileage GT350 take-out rear end, purchased it for $750.00 + $150.00 shipping.

    The rear-end build went relatively well
    14 years as a Ford Registered Technician, and many years racing with the Ford 9” rears sure helped, but I had sold or let a lot of my shop tools get away. Without a shop press & bearing splitter, I had to get creative.
    Harbor Freight has a Bearing Separator Set that works well with a slight modification. (Item 62593)
    I used my 25% off coupon and it was a little over $42.00.
    This tool will pull the Yoke off the pinion, as well as pull the inner pinion bearing from the pinion.
    The modification needed on the splitter tool is to grind a little more metal from the clamshell splitter, with a (Dumore/Dremel, I used a Craftsman Pro rotary grinder) and grinding bit. The splitter needs to clamp under the pinion bearing, when tightened, without hitting/bending the pinion bearing cage. You have to grind a little from the sides of the splitter for clearance. I wanted to save my old pinion bearing without ruining it so I could later hone it out a little, so as to make a slip-fit setup pinion bearing. This is needed to be able to quickly change shims under the bearing when doing set-up (Pinion Depth).
    Honing out the inner (Large) pinion bearing to make a setup bearing is easy, here’s how:
    Use a round “Sanding”, -NOT Grinding, bit in the end of your die grinding tool.
    It takes about 5 minutes to evenly hone a couple of thousands out of the bearing to get it to: “Slip Fit on the new Pinion”. Believe me, this will save you time later.
    Another tool needed is a tool to hold the pinion flange from turning while removing, or torqueing the new pinion nut & crush sleeve. I easily made this tool, Here’s how:
    I used a handle I had from a conduit bender. Flatten about 3” on one end with anvil & big hammer, drill two 13/32” holes, 1 + 13/16” apart in the flat end to match hole spacing on pinion yoke, that’s it.
    If you don’t feel like making one, you can buy a Pinion flange holder at for $21.99, or for $39.99

    The other problem I overcame was how to hold the diff: (Torsen) & ring gear ASSY to replace and torque the new ring gear bolts. I had used a hydraulic press in the past, as well as a heavy duty big vice, but now I had neither.
    You really do not want to clamp the Torsen in a big vise, as the barrell on the Torsen is quite thin.
    I went to Home Depot and for $3.50 I bought the wood needed to build this holding fixture (Pictured), that I designed. It worked like a champ. I also had some deck screws, and 3” C-Clamps that I used.

    Another method you can use to work with the ring gear bolt (100 lbs) torque is to use a long prybar as shown below.

    On disassembly I noticed the factory had marked the diff carrier caps with yellow paint stripes on each side. I always mark them myself with number stamps anyway, but may not be necessary.
    After all parts were cleaned with solvent/blow dried I began the assembly/set-up. What pinion shim to start with was my concern. I have used many pinion depth tools in the past, but I ALWAYS relied on gear-marking compound, and rolling a pattern for my final settings. If I would have had a professional Pinion Depth checking tool today, I could have set it at the (2.259 marking on the end of my new pinion.) I called my Master Mechanic friend at the Ford garage to ask if he had done any newer IRS Mustang rear ends and what pinion shim he ended up with. I also contacted US Gear Tech, as well as Strange Engineering Technical. No-one could give me an answer, other than start with what came in it. So that’s what I did.
    Started with (Original): .027 Pinion shim, .273 Left Side Carrier Shim, and .279 Right Side Carrier Shim.
    I have set up hundreds of rear ends, and I knew right away this was WAY-OFF. Only had about .003 backlash and the pattern looked like crap. (Way low on drive side, and Way high on coast side)
    Here is a link to a very good discussion of Ring and Pinion Setup and other topics:
    I must admit I am somewhat of a perfectionist, so I ended up changing the pinion shim and Carrier shims 8 or 9 times to get the pattern I wanted. Ended up with .037 Pinion Shim, .261 Left Side Carrier Shim, .291 Right Side Carrier Shim, and .008-.009 backlash. Pattern was near-perfect.
    TIP: To save time when changing shims, and checking patterns I did make a 2 + 1/8” long spacer, out of 1 1/4” PVC, then grind a little from the inside on one end. This eliminated me having to draw the Pinion Yoke on every time, as the yoke is a press fit, and the spacer was a loose-fit. The outer pinion bearing on this rear end is already a slip-fit so everything was fast/easy to change. Do not use crush sleeve for this step.
    For the final assembly of the Pinion, (I did replace both press-in cups in the housing) They are easy to replace, you can use a long drift punch, working evenly, side to side)
    TIP: On the inner pinion bearing final assembly. For those who don't have access to a press, here are two options that will work on getting the inner pinion bearing pressed on the pinion shaft: (1) (Put the pinion gear in the freezer over-night (or at least 8 hours) before assembly, and then 15-20 minutes before assembly, heat-up the inner pinion bearing in the oven to 375 degrees. The frozen pinion, and the heated bearing make it very easy to tap the bearing down on the pinion, it just about falls on. (Don’t forget to wear gloves, and put pinion shim on first). (2) Buy a 2" threaded black pipe, 8" long, and a 2" pipe cap, at Lowes or hardware store. This will make a perfect bearing driver to install the pinion bearing. Always use new outer bearing, seal, crush sleeve and pinion nut for final assembly.
    One other thing about the Pinion Flange, while it is off, paint the dang thing. (I don’t know why Ford doesn’t paint them). I had to use Naval Jelly to remove rust, and wire brush to prep for painting. I masked off seal area, and splines, and used silver header (High Heat) spray paint.
    Rear End-Final.jpg

    UPDATE: Here is Fords 'Super 8.8' newer Pinion Flange... "It's Painted" !
    Did they take that suggestion from me?... Probably NOT

    Now for the Swap:

    Ford has a great guide for the swapping, and bolt torque specs. I actually found it in the Ford Performance GT350 Diff Cooler install instructions. Most of the steps are the same. But what was important, none of the rear-end remove/install video’s I examined were specifically for the GT350 (Only regular Mustangs). The GT350 is slightly different, especially in the emergency brake area.
    Ford Instructions link:
    I put my car fairly high-up on 4 jack stands, using an 18” 2x4, or the fixture I made (below) you can then lower/remove the exhaust by yourself quite easily with a floor jack. (Put the 2x4/Fixture under the exhaust just in front of where the middle bracketry is welded on the exhaust.)
    Final Exhaust Fixture.jpg
    While it was off, I cleaned and painted the entire exhaust system with silver (High Heat) spray paint (masking off the two-stage muffler electrical cutouts). I also used a 12” 2 x 4 and floor jack also to remove and install the rear end housing back in the car. (Don’t try to handle it by hand, it weighs about 120 Lbs.)

    TIP: Removing Half Shafts. The pry-bar method does Not always work so well. I pried and pried on mine with 2 long pry bars, NO Luck, they would not budge. Also prying on them next to the seal area may damage the seal. The trick to popping them out of the diff is to use a sudden blunt force. I still had a “Tire Bead Breaker” slide hammer tool that I ended up using to EASILY pop out axles. If you can’t get this type of slide hammer tool, consider using Long ½” extension and big hammer. (Place end of extension on big round metal half-shaft housing, (Right where it comes out the side of the diff). You will have to hit it a slight angle, as you can’t hit it straight on. It should pop out easier than prying on it.

    Gear Oil and Cooling concerns: I filled the rebuilt housing thru one axle hole with a small funnel, before installing. This rear end spec says 1.5L -Without cooler, (I do not have cooler). That is about 51 oz. + the friction modifier. I wish Ford had designed this rear end to hold a little more fluid, as the Older 8.8 Mustang rears held 2.7 quarts. Anyway, I ended up using 60 oz. (I didn't add any Friction Modifier). The fluid level was right at the bottom of check plug hole when I checked it after installing. Keep it level when installing, and no fluid will leak from Axle openings.
    If you want to see a pretty cool thread on my rear-end cooling solution see the following link:‘super-8-8’-rear-end.117720/

    Now for the Comparison/Evaluation:
    This is not a huge change, it is a minor change.
    If your cruising in 5th gear at 70 or 80, and punch it you definitely feel faster acceleration.
    I was somewhat surprised to learn that the Speedometer/RPM was not off after swap, (It remained accurate)
    I still plan to use Forscan software, and a OBDLink Tool (426101) to change my BCM Data at: 726-12-01, From: 0175 -3.73 Axle Ratio, To: 019A -4.11 Axle Ratio.
    I understand that this setting has absolutely No affect on Mustangs with Manual transmissions.
    It does affect Mustangs with Automatic transmissions (It affects the shift points)

    Here is a chart of the comparison of both gears, speeds, and approx. RPMs.

    3.73 Rear Gear
    Trans Gear : Speed (MPH) : Engine RPM
    5th : 50 : 2250
    5th : 60 : 2750
    5th : 70 : 3150
    5th : 80 : 3650
    6th : 50 : 1400
    6th : 60 : 1700
    6th : 70 : 2000
    6th : 80 : 2250

    4.11 Rear Gear
    Trans Gear : Speed (MPH) : Engine RPM
    5th : 50 : 2500
    5th : 60 : 3000
    5th : 70 : 3500
    5th : 80 : 4000
    6th : 50 : 1600
    6th : 60 : 1900
    6th : 70 : 2200
    6th : 80 : 2550

    Overall this was a Great, Challenging and Fun project.
    Having driven with the new setup some 400 miles now, the Gas mileage remains at 18-21 MPG on the highway, 15-17 in town.
    Am I glad I did it, and happy with it? Absolutely!

    My next Venture?
    a 4.56

    To read about my Gear Change from 4.11 to 4.56
  2. Mjc1241

    Mjc1241 Well-Known Member

    First Name:
    2016 Shelby GT350
    Jun 25, 2017
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    Great write up John. Looking forward to impressions after more miles.
  3. cosmo

    cosmo Well-Known Member

    2005 Mustang GT
    Sep 22, 2014
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    I feel like that would really help out first and second gear, and make it feel peppier. Does it?
  4. firestarter2

    firestarter2 Well-Known Member

    RR G350 Track
    Jun 16, 2016
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    Can you do the math for 1 through 3rd?
  5. Epiphany

    Epiphany Well-Known Member

    I like to disassemble things.
    Oct 11, 2015
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    Philanthropist and ballroom dancer extraordinaire
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    By far the best writeup I have seen to date. Great work!
    JohnVallo likes this.
  6. sublime1996525

    sublime1996525 Well-Known Member

    2017 Race Red GT350
    Sep 17, 2016
    Ricers make baby Jesus cry, Utah
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    Damn good write up!! :cheers:
    JohnVallo likes this.
  7. Zombo

    Zombo befejezett

    2016 GT350 Track Pack - Race Red, Blk Roof - G5405
    Apr 1, 2016
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    Excellent write-up!

    Regarding strength, assuming the materials are the same and the diameters of the ring and pinion are roughly the same between the 2 sets, the 4.11:1 (9/37) should be stronger than the 4.09:1 (11/45). Because less teeth are distributed about a similar diameter, each tooth has to be thicker, hence stronger. From a noise perspective, generally, the opposite is true. The more teeth, the finer the mesh, the less backlash and noise.

    On a related topic, ever wonder why gear ratios normally have odd numbers? (I realize many here know the answer, I've just never seen it discussed so I figure it might be a helpful discussion for others).

    Lets say we have an even 4.00:1 ratio; that could be 40 and 10 teeth on the ring and pinion, respectively. When creating the gear teeth, due to manufacturing tolerances, the final gear tooth on both is usually somewhat out of tolerance due to the accumulation of tolerances during fabrication. In the 4.00:1 example, the "bastard" tooth on the ring gear would mesh with the same tooth on the pinion (with every revolution of the ring, and every 4 revs of the pinion), causing excessive noise and wear. The bad pinion tooth would mesh with the same 4 teeth on the ring, causing the same wear/noise issues.

    If you change the ratio to something uneven like 4.11:1 (9/37 like) the bad tooth on both the ring and pinion mesh with all the teeth on the mating gear, thus distributing the wear evenly.
    DocWalt, sublime1996525, rj45 and 3 others like this.
  8. Strokerswild

    Strokerswild Shallow and Pedantic

    First Name:
    Stuff With Wheels
    Nov 7, 2014
    Southern MN
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    Based on my one very brief stint behind the wheel of a 350, that gearset should have came from the factory. Or at least have been an option. :thumbsup:

    Or even a 4.56 (or thereabouts). :headbang:
    Hack likes this.
  9. GreenS550

    GreenS550 Well-Known Member

    First Name:
    2019 Mustang Bullitt K3107 Whipple 2018 Mustang GT Premium A10 Ruby Red
    Nov 18, 2017
    Houghton, MI 49931
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    When I put the Vortech on my Bullitt it made similar rwhp 488 right near the top of the engine rpm limit. However it came on more gradually then say a roots type. Of course it would only do about 6800 rpm with that 4.6. So I changed the 373s to 410 and it really helped everywhere. The gas mileage was about the same but 5th now became a more usable hwy gear. So, my guess is that it would really make a difference with the GT350 with that huge redline you GT350 guys have.
  10. Zitrosounds

    Zitrosounds Well-Known Member

    First Name:
    16 GT350R/16 GT350TP/15 GT-PP/12 GT-PP
    Oct 20, 2015
    Madison, AL
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    #10 Zitrosounds, Jan 11, 2018
    Last edited: Jan 11, 2018
    Thanks for the information. That makes two know that I know of that have done the U.S gear swap. I personally have not felt the need for a gear change but can see for those that want a quicker initial response especially in real world street driving. Now all that is left is some timed test data to show the differences. That will really hep those looking to make the swap.
  11. ctandc72

    ctandc72 Well-Known Member

    '19 GT 6 speed Base
    Nov 1, 2017
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    I did some digging before I swapped my 3.15 gear to 3.55 in my V6 car. Apparently the ABS / wheel sensor is what reads wheel / tire speed now. Any small increment gear changes are automatically 'corrected' without the need to change anything...GPS verified correct speedo with 3.15, then after with 3.55. Even so I did change the correct line in FORScan....though I really didn't need to.
  12. Hack

    Hack Well-Known Member

    Mustang, Fiesta
    Nov 26, 2014
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    Awesome write up. Very impressive.

    I'd like to have a better gear ratio for street driving but after seeing the math on the 4.09 that Ford used to offer I decided it wasn't worth it for me. And now after having the rear diff out for retrofitting the cooler I really don't want to swap rear gears back and forth every time I go to the road course. It's more of a pain to tear apart so the slight improvement wouldn't be worth it to me.

    I think probably an entirely different manual transmission could be a better answer. Something with a deeper first and somewhat deeper second at least. I would also prefer a less tall overdrive for sixth. Not that I would spend the money, the car is great for the road course as is. I also wouldn't want to swap transmissions every time I go to the track either. I've owned some Mustangs in the past where a transmission swap is an hour job, but the GT350 is much more of a pain to get the trans in and out. Just my thoughts.
    JohnVallo likes this.
  13. PP0001

    PP0001 Well-Known Member

    2015 GT350 Track Pack - 2018 GT350R Base Model - 2020 GT350R Heritage Edition Package On Order
    Jun 9, 2015
    Valley Of The Sun
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    John, first of all you need to be commended on your outstanding write up which includes a tremendous attention to detail based on your many years of automotive experience in the real world. :thumbsup::thumbsup:

    Secondly, based on improving the gearing on your GT350 by ~10% I would suggest that you can feel a definite difference in acceleration between the stock gear ratio and your new gear ratio and especially in the first 3 gears?

    Lastly, assuming that you are presently retired I would suggest that you come out of retirement and offer your vast experience and services to some of the automotive magazines and/or local Ford dealerships as they could certainly benefit from even more HP initiatives.;)

    Thanks again for an outstanding write up!

  14. RyanR3KC

    RyanR3KC Well-Known Member

    2018 Camaro SS 1LE
    May 19, 2014
    Johnston, Iowa
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    would love to see how feels on a road course. The 1LE had 3.91s and she was great.
    JohnVallo likes this.
  15. MrCincinnati

    MrCincinnati Well-Known Member

    2017 GT350R
    Jun 20, 2017
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    I wonder is he able to get any more utility out of 6th with the change or is it still so tall that top of 5th is max speed.

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