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Supertop & Gas Mileage.....

Discussion in 'Intermediate CJ-5/6/7/8' started by Lockman, Jul 22, 2019.

  1. Jul 22, 2019
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Silver Hill,...
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    So... Here's one to ponder : Took my CJ5 for a highway ride. Very pleased that she Ran fantastic , roundtrip.
    First 80 miles @ 60 MPH , west wind ~ 15 mph , with the top on & doors & windows off. Got 13 MPG
    :cry:

    Return trip , top down , cinched & stowed. 80 miles @ 62 MPH, tail wind ~ 10 mph. Got 20 MPG ? :whistle:
     
  2. Jul 22, 2019
    Jeepenstein

    Jeepenstein Me like Jeep..

    North Central FL
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    Jun 15, 2003
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    Square box, round wind?
     
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  3. Jul 22, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
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    Did you have an precise measurement of the gas used each way?
     
  4. Jul 22, 2019
    Lockman

    Lockman OK.....Now I Get It . 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Silver Hill,...
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    Yes Sir.......6.1 Gallons , 1st leg. 4 gallons , return .
     
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  5. Jul 22, 2019
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    So you had a 15 mph headwind going and a 10 mph tail wind returning? The wind resistance (drag) is a mighty barrier... it increases like the square of the velocity. Add the headwind/tailwind to your speed, and wrt drag you had a 75 mph speed on the first leg, and a 52 mph speed returning. (75^2 - 52^2) / 75^2 = 0.52, or a 48% reduction in drag on the return trip. If your gas mileage only depended on drag, you would expect a =1/0.52 = 1.92 (92%) improvement in gas mileage, or roughly doubling. You didn't quite make that, instead making a 20/13 = 1.54x (54%) improvement - drag is a major thing in gas mileage, but not the only thing. Possible that top up or top down made the situation better or worse; with the wind effect, you cannot make any conclusion.
     
    Last edited: Jul 22, 2019
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  6. Jul 22, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
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    Dang, you lost 7 mpg bucking a 15mph wind?
     
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  7. Jul 22, 2019
    Twin2

    Twin2 not him 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Virginia Beach, VA
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    I always estimated my jeep V6 . only got 12mpg ever
    so if you got 20 your way ahead (y)
     
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  8. Jul 22, 2019
    kenb

    kenb Cleverly disguised as a responsible adult

    Detroit
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    I think there is something to it. When I switched from hard top to soft top on my wrangler this spring I noticed it felt like it needed more throttle to maintain speed on the highway. All that fabric moving around must create some drag.
     
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  9. Jul 22, 2019
    Beach66Bum

    Beach66Bum 1966 Tuxedo Park Mark IV 2024 Sponsor

    Big Island on...
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    Same here, even all tuned up. I blame the 4.88’s and no OD.
     
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  10. Jul 22, 2019
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Yeah, (13 + 20) / 2 = 16.5 mpg which is phenomenally great for a CJ. I would be very pleased with that. Frontal area does not change much, top up or down. I would not be surprised at a small but detectable difference depending on the top folded or not, but IMO the conditions were not well enough controlled to say anything about that from this data.
     
    Last edited: Jul 22, 2019
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  11. Jul 22, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
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    A soft top would be less aerodynamic by design so it seems logical to me.
     
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  12. Jul 22, 2019
    Jw60

    Jw60 Sitting up n buckled down. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Sedalia MO.
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    Always drive 62mph. :schooled:
     
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  13. Jul 22, 2019
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
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    I got a pretty consistent 18mpg on my cross-country trip until I got close to home and put the hammer down. :twist:
     
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  14. Jul 22, 2019
    45es

    45es Active Member 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Naches, WA
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    Huh? How on earth did you come to that conclusion? :shrug: Soft top or not, you are still driving a BRICK.
     
  15. Jul 23, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
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    :lol: Just because of the hump of the bows basically....center and back bows would create a drag compared to the flatness of a hardtop. But yeah, the Jeep front and windshield are certainly more of a drag yet.
     
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  16. Jul 23, 2019
    Hellion

    Hellion Regurgitated

    Eastern TN
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    Next trip, do the drive with the windshield folded down and note the MPG. :schooled:
    A major part of the 'brick' is that big flat windshield, no?

    Be sure to wear some nice aviator goggles.
     
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  17. Jul 23, 2019
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    I've always noticed a gain in power with the windshield folded down.
     
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  18. Jul 23, 2019
    Hellion

    Hellion Regurgitated

    Eastern TN
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    Correction: No top and windshield down and with or without a roll bar; e.g. run what you brung.
     
  19. Jul 23, 2019
    william_cj3b

    william_cj3b 3BOB driver

    Milton, FL
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    Spoken as only someone well versed in 4-cylinder Willys jeeps could :beer:
     
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  20. Jul 24, 2019
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    Also hunch down behind the wheel & wear one of these-


    [​IMG]


    & one of these-
    [​IMG]


    :)
     
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