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Thread: lowering the final drive, aka "nerd gear" for better fuel economy/MPG

  1. #1
    Senior Member MightyMirageMpg's Avatar
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    lowering the final drive, aka "nerd gear" for better fuel economy/MPG

    ide like too discuss a few options, and the problems they create, with the end goal of lowering the final drive for better efficiency.

    anyone who's ever driven 40mph in 5th gear on a flat road in a "non-uk spec" mirage, and reset the mpg meter, will tell you that its very easy too maintain over 70mpg, even over very long periods of steady state travel. obviously, there is gains too be had here.

    idea #1. custom made set of gears for final drive.

    I've emailed a few companies and received "General prices" of gear sets. i was told by multiple companies, "were not interested" but two companies have given me similar pricing. $3000 for a 1-off set, or $3600 for 3 sets. ($1200/ea)

    problems: pricing. i figure even at a large efficiency gain, the pay-off is near 100,000 miles. you must remove and disassemble your transmission. speedometer will be off.

    idea #2 uk-spec final drive swap, us spec 5th gear

    problems: i am unable too locate a source for the parts, and again, removal of transmission, disassembly, and your speedometer will be off.

    idea #3. complete transmission swap

    problems: well there's obviously lots, but "its been done before" and I'm positive it can be done, but the time, labor and cost will be huge for the first man on the moon.


    questions i have,

    can anyone locate the part number for the uk-spec final drive gears?

    does anyone understand how our cable-less speedometer works, enough so to say they would be able too correct it?

    is there anyone who would be interested in getting custom gears made? I HAVE A SPARE TRANSMISSION that can be the donor for the company making them.

    anything I'm overlooking? there should be no computer issues, correct?
    yes i realize taller tires will help, but i can't justify sticking 40" mud boggers on the mitsu and claiming its for mpg.


    Last edited by MetroMPG; 05-12-2016 at 06:42 PM. Reason: (bolded some text, added to title)

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  3. #2
    Administrator Daox's Avatar
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    Searching for a 'speed sensor' only comes up with ABS sensors. There also isn't any speed sensor in the transmission (the only sensor is the reverse switch). So, there might not be anything in the transmission you even have to worry about.
    Custom Mirage products: Cruise control kit, Glove box light, MAF sensor housing, Rear sway bar, Upper grill block

    Current project: DIY Nitrous oxide setup for ~$100

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 47.2 mpg (US) ... 20.1 km/L ... 5.0 L/100 km ... 56.7 mpg (Imp)


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    MightyMirageMpg (05-12-2016)

  5. #3

    Question final drive swap... 5% improvement in my Firefly

    I think the 1.2L Mirage is screaming (literally) for this mod, for the fuel economy minded.

    I've done a final drive swap to my own car (1.0L, 3-cyl Pontiac Firefly a.k.a. Chevrolet Metro) -- I swapped in the transmission from the 4-cylinder car with a numerically lower final drive (but all other gears the same) to get the "taller" gearing = lower cruising RPM and improved fuel economy.

    Here's the Mirage's 5th gear RPM difference between the UK and non-UK spec final drive with the non-UK 5th gear

    Related thread: Gear ratios: 2014 Mirage transmissions, 5-spd manual & CVT (speed vs. RPM chart)

    Using these values:

    5th gear: 0.804
    165/65R14 tire diameter: 22.4 in. (calculator)

    Speed
    (MPH)
    Engine RPM
    (4.055 final drive)
    Engine RPM
    (3.55 U.K. final drive)
    Difference
    40 1956 1713 243
    50 2445 2141 304
    60 2934 2569 365
    70 3423 2997 426
    80 3912 3425 487
    These values from this calculator


    My experience


    Bigger gearing change: The gearing change I made to my car dropped the RPM quite a bit more than this. EG. at 50 mph, the engine is now turning 485 RPM lower than stock, vs. 304 RPM lower in the Mirage calculations.

    I tested the difference, and it worked out to about a 5% improvement in fuel economy over a range of constant top gear speeds:

    Name:  mpg-vs-speed-chart-z-b4-aft.gif
Views: 2464
Size:  22.3 KB

    (Note: the improvement looks better on this chart than it actually is because the "after" tests were done in warmer weather, which improves efficiency even when nothing else changes. That's why I calculated the benefit to be closer to 5%, not the ~8% you see here @ 50 mph.)

    Full details: Project 'nerd gear': taller tranny transplant nets +5.2% MPG - MetroMPG.com

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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    Senior Member MightyMirageMpg's Avatar
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    interesting facts, the 3a9x and the 4a9x engines were developed together, meaning you could (possibly) mate the 2008+ lancer base model trans to our engines and the opposite would apply to those who want an all aluminum 4cyl.

    while this is definitely not a weekend bolt in and go operation, it's definitely something to look into, and hopefully confirm.

    sadly, the only information i found regarding the lancer transmissions leads me too believe the overdrive isn't there, either, and this would probably best suit the people seeking more power.

    i also remembered another member mention the smart fortwo turbo shares the engine block, but they are even worse off than us, with a numerically higher final drive, and nothing but semi-manual transmission that, if googled, leads me too believe it's nothing but undesirable.

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    LetItMarinate (01-08-2019)

  9. #5
    In the future (when the cars are out of warranty) I could see some more traditional "performance" oriented UK owners willing to swap non-Euro final drives for better acceleration.

    ---

    Ideally though, the best approach would be to find a taller 5th gearset that fits the Mirage transmission. In some cars, swapping 5th is as easy as unbolting the stamped steel cover on the end of the gearbox and taking 2 nuts off to change the cogs. If I'm not mistaken there are a couple of GM/Saturn gearboxes with different 5th gear ratios that are interchangeable 5th gears.

    That way you get the fuel economy benefit in top gear, but preserve the factory acceleration in 1-4.

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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    At least in Toyota's world, the Corolla / Matrix / Celica transmission won't bolt right up to a Tercel / Echo / Yaris block. But, the Corolla / Matrix / Celica transmission will bolt up to the the bellhousing of a Tercel / Echo / Yaris. This greatly increased the number of transmissions to choose from. They were also beefier which a lot of the turbo guys liked because they were snapping differential bits. Perhaps something to look for on the Lancer?

    Another thing I noticed but never tested, is the differential gearing from the automatics was WAY taller than the manual transmissions. While they weren't a direct drop in, the automatic differential from a Tercel (thats where I have most of my info from) I believe would drop into a manual trans from a Corolla. The difference was going from 3.722 differential ratio to about 2.821, so it was pretty huge.

    So, these are some ideas from another manufacturer. Not sure if they'll help, but they're ideas!
    Custom Mirage products: Cruise control kit, Glove box light, MAF sensor housing, Rear sway bar, Upper grill block

    Current project: DIY Nitrous oxide setup for ~$100

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 47.2 mpg (US) ... 20.1 km/L ... 5.0 L/100 km ... 56.7 mpg (Imp)


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    Quote Originally Posted by MightyMirageMpg View Post
    anything I'm overlooking?
    I'm curious to see if any dealer would be willing to give this a go.

    Quote Originally Posted by MightyMirageMpg View Post
    yes i realize taller tires will help, but i can't justify sticking 40" mud boggers on the mitsu and claiming its for mpg.
    We need photoshop to make this happen.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 manual: 49.6 mpg (US) ... 21.1 km/L ... 4.7 L/100 km ... 59.5 mpg (Imp)


  12. #8
    The potential downside to going with taller wheels/tires comes from changing multiple variables in addition to lowering RPM a little.

    I have read that it has worked for some people on some cars, but there's no guarantee, because:

    -- Taller ride height = increased aerodynamic drag

    -- Rolling resistance will be different, maybe better, maybe worse

    -- A wider tire adds to frontal area and drag coefficient = again increase aerodynamic drag

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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  14. #9
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    Quote Originally Posted by MetroMPG View Post
    Here's the Mirage's 5th gear RPM difference between the UK and non-UK spec final drive with the non-UK 5th gear
    I wonder if it would be much easier to simply swap a UK tranny in? I've messed with trannies before but the complication and specialty tooling required to swap out gearsets pretty much ensures only a few select people would be able to do a swap like that.

    Edit: MightMirageMpg's third idea apparently. I see I can get a 1.2l off ebay for around $700 shipped with under 20k on it... if we could get a source for UK trannies that would be ideal!
    Last edited by Littlestan; 05-13-2016 at 09:00 PM.

  15. #10
    You get a smaller RPM drop if you swap the whole transmission, because the UK 5th gear is different than the non-UK one.

    5-speed manual (F5MDB)

    1st - 3.545
    2nd - 1.913
    3rd - 1.310
    4th - 0.973
    5th - 0.804
    Reverse - 3.214
    Final drive - 4.055
    UK 5-speed manual (no part number available)

    1st - 3.545
    2nd - 1.913
    3rd - 1.310
    4th - 1.027
    5th - 0.850
    Reverse - 3.214
    Final drive - 3.550

    It would be easier though.


        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)


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