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Thread: Is the Automatic Transmission the culprit?

  1. #1
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    Is the Automatic Transmission the culprit?

    I have two '97 1.9 roadsters. The one I have had for a few years is an automatic, R/H drive, from Japan. I bought the second one a couple of months ago in New York and drove it to British Columbia by way of Texas and San Diego, a distance of about 5500 miles. Most of the distance was freeway driving, avg 70-75 mph in fifth gear. I got about 30 mpg (US gallon). Of course, the automatic one is in kilometers, but after conversion, the gas mileage I get is about 20 mpg. In a quest for better gas mileage I have replaced the plugs, ignition wires, and distributor, (with little or no effect) The transmission is a GM. Is this a normal spread when comparing gas mileage between std. and auto transmissions?
    The Future ain't what it used to be.

  2. #2
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    My old 318ti with the same powertrain did 25, and I was not kind to that hateful car. I would dig deeper. Did the TCC work?

  3. #3
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    Not a normal spread from anything I've ever read.

  4. #4
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    Quote Originally Posted by WWPierre View Post
    I have two '97 1.9 roadsters... an automatic... bought the second one a couple of months ago in New York and drove it to British Columbia... about 5500 miles... freeway driving, avg 70-75 mph in fifth gear. I got about 30 mpg... the automatic one... is about 20 mpg... Is this a normal spread when comparing gas mileage between std. and auto transmissions?
    I could be a normal spread from a road trip in the heat, to stop and go driving with stops for errands, in the Canadian winter.
    My 1.9 manual gets 30 mpg on my short trips, but that falls to 24 mpg in winter suburban pleasure drives. If I was to use the Z3 for the kind of daily winter driving my Subaru gets, I would not be surprised at 20 mpg.
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  5. #5
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    Not sure about 1.9s, but manual and automatic 2.8s have a different diff ratios, causing automatic to use more gas (~10% on average) even in straight line.

  6. #6
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    Overall top ratio:

    Auto = 3.21
    Man = 3.45

    Find a different scapegoat.


    /.randy

  7. #7
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    Quote Originally Posted by rf900rkw View Post
    Overall top ratio:

    Auto = 3.21
    Man = 3.45

    Find a different scapegoat.
    Where did you got those numbers? Diff ratios:

    1.9 auto = 4.44
    1.9 manual = 3.45

    2.8 auto = 4.10
    2.8 manual = 3.15
    Last edited by deni2s; 01-11-2018 at 03:51 PM.

  8. #8
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    Not diff ratio, OVERALL ratio. Diff X Trans ratios is what matters.

    4.44 X 0.723 = 3.21

    3.45 X 1.00 - 3.45

    The automatic is geared taller.


    /.randy

  9. #9
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    Which means on freeway driving 1.9 automatic should theoretically get even more mpg than manual...

  10. #10
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    It seems that for any car, the automatic version is always geared taller, makes fewer rpms per mph, and gets better mileage.
    My 2006 Scion xB manual may be an extreme example:
    Manual -- Combined Final Drive (5th gear) = 3.514 making 4090 RPM @ 80 MPH.
    Automatic -- Combined Final Drive (4th gear) = 2.910 making 3410 RPM @ 80 MPH
    At any speed, the automatic RPM in 4th gear was 17% less (680 rpm) than the manual in 5th gear.
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  11. #11
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    If all that mattered was ratios, then yes, most automatics would win. But there are many more factors involved. Weight. Efficiency ( auto has power sucking hydraulics). And then there is that thing doing its job so well, converting torque into heat. The torque converter alone is a major killer of mileage, and why a properly functioning TCC system is important.

  12. #12
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    Quote Originally Posted by Vintage42 View Post
    It seems that for any car, the automatic version is always geared taller, makes fewer rpms per mph, and gets better mileage.
    My 2006 Scion xB manual may be an extreme example:
    Manual -- Combined Final Drive (5th gear) = 3.514 making 4090 RPM @ 80 MPH.
    Automatic -- Combined Final Drive (4th gear) = 2.910 making 3410 RPM @ 80 MPH
    At any speed, the automatic RPM in 4th gear was 17% less (680 rpm) than the manual in 5th gear.
    Up until somewhat recently, autos got worse mpg than manuals, not betrer. As Randy mentioned RPM is not all that matters, the torque converter sucks up power sapping mpg away. Looking at fuelly, manual scion xb's have higher mpg on average. This should be the same for the z3, I would expect a manual to get better mpg, but only by 1-3mpg max.

  13. #13
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    Back when I was looking at newer cars, the manuals always had a better mpg rating. Toyota tends to lag behind... basically everyone in terms of improving efficiency etc, so they should still be a relatively good example of automatics getting worse mileage than manuals without having to dig particularly deep. Usually they do weigh more and the torque converter loses a lot of energy to heat via friction (I think my material said friction from the fluid being directed around its paths), as Randy has noted already. Modern autos are significantly better, but the 4L30E is basically a TH180 with an extra section tacked on... And the TH180 was developed in '69 or something, which makes it slightly more modern than my parents.

  14. #14
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    Quote Originally Posted by BimmerBreaker View Post
    Up until somewhat recently, autos got worse mpg than manuals, not betrer... .
    We are comparing highway miles to overall miles. I was using a specific example of cruising in top gear in a 2006 xB, where a 680 rpm difference gave the automatic better mileage.

    I agree that for mileage, automatics are now as good or better than manuals.
    For instance, in 2015 the manual xB was rated at the same overall mileage as the automatic = 22/28 for both:
    https://g.co/kgs/j6ibUf

    In 2017 the BMW 320i is rated essentially the same for both:
    Automatic = 23/35/28
    Manual = 23/35/27
    https://www.fueleconomy.gov/feg/bymo...3_Series.shtml

    For the 2017 Mazda6, the automatic is better:
    Automatic = 27/35/30
    Manual = 24/34/28
    https://www.fueleconomy.gov/feg/bymo..._Mazda_6.shtml

    And for the 2017 Honda Accord, the difference is dramatic:
    https://www.fueleconomy.gov/feg/bymo...a_Accord.shtml

    As I said in #10 think that for any car these days, the automatic is rated the same if not better than the manual. Especially the Hwy rating where the gearing of the automatic or CVT is taller.
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  15. #15
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    You are mixing many technologies and transmission types into the broad band of "automatic". For instance Honda has never made a true automatic, only hydraulically shifted manuals. DCTs are now popular and commonly called an automatic. Computer shifted manual.

    Bottom line is the 4L30 is a classic GM design from the 60s with a bit of early 80s grafted in. It will NOT ever be an efficient unit. But the OPs mileage is still sub par, even accounting for the lump.
    Last edited by rf900rkw; 01-13-2018 at 09:36 AM.

  16. #16
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    On that note, totally change the fluid. Might help delay the development of issues like solenoids sticking.

    When you drive across the country, you sample gas qualities that might vary pretty decently. Plus changes in elevation and ambient temperature.

    In New Mexico, I could get 30+ mpg uphill with passengers and the a/c blasting; in the same car, flat terrain, no passengers or a/c, I was getting 20-24 mpg in TX. That may be part of the issue.

    Have you made a list of factors that change fuel mixtures? You can do that and go down the list and check components for wear and faulty operation.

  17. #17
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  18. #18
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    That is the old automatics. The 4-speed auto in my 2008 Subaru could be the worst.
    The 8-speed automatics in the new BMWs etc are better in every measurable respect than manuals, except pleasure.
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  19. #19
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    Quote Originally Posted by rf900rkw View Post
    You are mixing many technologies and transmission types into the broad band of "automatic"... Honda has never made a true automatic, only hydraulically shifted manuals...
    I was just saying that nowadays automatics get as good or better mileage than manuals.
    The lady across the street has a 2015 Accord automatic and I don't know if it was hydraulically shifted, but it does shift by itself.
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  20. #20
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    The new autos are simply amazing improvements. We're doing incredible things these days.

    Unfortunately, the 4L30E was basically ready for retirement before it was even implemented. That's how I feel, anyway.

  21. #21
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    I know what you mean. I have the 4L30E in my 528i. Luckily in a larger car like that it does a pretty good job. Overall combined City/Highway for me is 24.1 MPG.
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