Installed on MY09/10/11 Boxer Diesel models.

Gear Ratios

Gear Ratio
1 3.454
2 1.750
3 1.062
4 0.785
5 0.634
6 0.557
R 3.636

These ratios were used/confirmed here: Engine Speed vs. Vehicle Speed (valid for Impreza 2.0D at least).


  • Fluid capacity: 3.7 liters according to car manual.
  • Same length as 5MT 1
  • +5 kg mass vs. 5MT 1
  • “Improvements featured in the new transmission include a cable-type linkage and a carbon synchronizer for the first and second ratios, constantly-engaged synchromesh for reverse gear, to improve the shift feel, and optimised lubrication paths for reduced friction losses.” 1

1) http://europeanmotornews.com/2010/10/27/2011-subaru-outback/


16 responses to “6MT

  1. Daniel Holdforth

    Hi Subdiesel,
    I’m posting here as you would seem to have (from my searches) the most extensive knowledge for my particular dilemma.
    I have an ’08 Outback (UK spec) that suffered a crank failure. I sourced a written-off 2013 Forester and acquired the engine/6MT/diff and transplanted them into my 08.
    Although the car runs, I have 2 ecu problems to solve.
    The ECU doesn’t like the new injectors as they have different flow characteristics. I expect this could be tuned out but can’t be sure so will likely fit the old injectors (what fun!)
    Bigger issue I think is that the torque maps are expecting different RPM/wheel speeds than it’s seeing and this seems to be triggering some strange reduced power modes but not triggering any warning lights.
    Do you know if the 5MT ecu can be adjusted to deal with the 6MT? I thought if the torque maps can be adjusted, the 2 – 6 ratio data could be inserted into the 1 – 5 table leaving the ecu confused only by 1st gear which it would likely ignore as clutch slipping at takeoff.
    What software would you suggest best for making these adjustments?
    Many thinks in advance.


    • Hi Daniel, sorry for the delay!
      First off, it’s been a long time since I did Subaru Diesel ROM analysis and related work (mostly MY09, some MY10).

      Gearbox, final drive ratio:
      I remember studying a function which calculated gear out of RPM and wheel speed inputs.
      As usual, two thresholds (low, high) are being used for comparisons. The function contained something like this:

      if (value > Threshold1Low AND value < Threshold1High) then gear = 1
      else if (value > Threshold2Low AND value < Threshold2High) then gear = 2

      If gear could not be determined or clutch switch was detected it would probably set gear = 0 and perhaps some additional flags. Not sure about those effects – did not investigate these – ROM software is simply too big to analyse everything with reasonable effort…
      Threshold values are usually single numbers stored in calibration area, related ones are often, but not necessarily, stored one after the other. Easy to change using ROM editor, once their location (address), data type and unit is known.

      In theory, 5MT ROM incl. calibration data could be modified into a perfectly working 6MT ROM – might require lots of diligent and cautious work though!
      Instead, your idea about “2 – 6 ratio data could be inserted into the 1 – 5 table” seems reasonable, much faster and safer as this results in only minor modification. Still, location of relevant data is needed.

      Currently I have no idea about the status of Subaru related open source knowledge like ECU definitions and software, forums etc. that happened after about year 2011 or so. I am certainly willing to learn, browse some info and help out if feasible.
      Now, can you log ECU parameters and read trouble codes, is your ECU supported by RomRaider, do you have your exact ROM file? These things are sort of per-requisites…

      I don’t know much, Euro5 vs. Euro4 injectors are most likely different. In my car’s ROM there were definitely some injector hardware specific maps as the ECU needs to know a bunch of characteristics to pre-calculate timings. Not only flow vs. pressure, also stuff for small injection quantities. For example, injected amount does not only depend on time span of applied voltage, but also on time span between those partial injections per stroke as injector needle might not fully settle within very short consecutive actuations.
      I also think, injector codes differ as well, Euro4 ROM probably won’t accept Euro5 style codes. Fitting the older injectors would solve that, using Subaru dealership (or maybe even open source nowadays?) software should allow to tell ECU those correct codes. This calibration should make engine running smoother by evening out injector differences between cylinders. On top of that, the ECU’s own “injector learning” function is meant for further improving idling – adjusting for tiny injection amounts.

      I recommend checking out RomRaider software and forums and see what it could do for your case.
      Feel free to post further questions! Ignore that I am currently using a different account on this site.



  2. Do you know what the differential ratio is? That gearing looks great for economy on the open road.


  3. Great! Thanks!


  4. What is the R/P for the 2012 6 speed trans?


  5. Thank you. That helps me out. I think the last bit of information I need is the ring and pinion ratio. I’ve heard it was a 3.90 and I’ve also heard it was a 4.44 for the 2012, but I’m not sure.


  6. Hi folks, I have a 2008 Subaru Outback 2.0D with a 5 speed and i’m thinking of swaping it for a 6 speed transmission from a later Subaru.
    So, please help if you can.
    1. Is it a straight swap? Lenght and all..
    2. Do you need an ECU from the 6 speed?
    3. Does it need to be from an Outback or can a Forester or a Legacy transmission do it?
    Any help is appreciated. 🙂


    • Hi, can’t help with such mechanical questions, don’t have part numbers, dimensions etc.
      ECUs from 6MT models are fitted with managed close-type DPFs unlike yours, the 2008/2009 first gen with unmanaged DPF. Apart from different software, ECU pinout, harnesses and perhaps CAN communications might also differ.
      I looked into all kinds of diesel ROMs, 5MT, 6MT and CVT versions, they all calculate gear from vehicle speed and engine speed using ratio threshold values defined in calibration data. Interestingly, even 5MT ROMs support 6 gears. I could help you adjust the necessary values so that gear determination works correctly after swap. Otherwise, gear 0 (zero) would be determined, e.g. not engaged or slipping clutch, probably limiting power. Haven’t done this yet, cannot guarantee perfect result as there might be other software things that should be changed as well. ROMs are too big to analyse and not miss something.
      I’d assume, all 6MT gearboxes are the same parts, just rear differential final drive ratio differs across models.
      To summarize, if the swap is mechanically feasible, I’d keep the ECU and just do custom calibration. Vehicle speed originates from VDC/ABS unit over CAN, reading wheel sensors, so no problem there.


  7. I’ve done this swap and all works well. I had an engine failure and got a full engine gearbox and diff from a 2013 Forester. Final drive ratio is different so you need to swap the diff as well. I had to do a bit of adapting of the gearshift mechanism as while it fits, it needed a cover made and a couple of tweeks to get it installed. For the engine, I swapped the injectors and wiring harness (no problem with the DPF) and run on the 08 ecu. I never got around to changing the ratios in the ROM, probably should. Covered about 50,000 miles since and all is good. Speedo reads accurate.


    • Cool, good to know.
      Ah yes, 6MT uses cables for shifting, 5MT levers perhaps?
      You are running on 2008 ECU including original Euro 4 ROM?
      If you can tell ROM version or send me a dump and want to log using RomRaider perhaps, I could tell you “gear” RAM address to log. Possibly zero all the time if ratios don’t fit within calibrated thresholds. On the other hand, might not matter for usual driving conditions.


    • Are the diesel diffs still an R160 or are they R180?


  8. Anyone know the r/p of the 2012 6 speed diesel transmission? Someone has to know.


  9. Hello,
    How much the rear differential ring and pinion ratio (6speed Transmission)?


    • Hi, final drive ratio depends both on model and transmission type.
      IIRC using same 6MT gearbox, Impreza and Legacy/Liberty used to have higher top speed, lower ratio than Forester/Outback/XV.
      Current model specs ex
      Final reduction gear ratio, 2.0D models: 4.444 (MT) / 4.111 (CVT)
      Specs for old Euro 4 Impreza 2.0D definitely list 3.900 and top speed 205 km/h.


  10. Hello 🙂
    So ive got a 2010 Forester 6MT. Runs amazing etc and had no issues. But because im wanting to get a custom airbox/turbo piping done through someone ive already sourced and said they can do the job. Im wanting to upgrade my intercooler to something like a process west or verticooler. Something upgraded i dont entirely mind. But im not sure what is going to fit. Would you be able to recommend any or know of intercoolers from certain cars that will fit? Im close to getting a custom exhaust and custom dyno tune after all this work but im not doing any of it till my intercooler isnt factory just to be safe. Im also not sure if a blow off valve could even be put on? Please help meeeee.


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