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Low Rpm At 45mph

Discussion in 'Intermediate CJ-5/6/7/8' started by Cornsipper, May 26, 2023.

  1. May 26, 2023
    Cornsipper

    Cornsipper New Member

    Memphis
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    May 26, 2023
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    3
    Hey this is my first post so here goes. Just got a 1977 cj5 with 232 3 speed t150. It is had really laggy take off power and when I am driving at 45 mph in 3rd gear it only is doing 1800 rpm. I realize this might be an engine problem but I am not 100% sure what the gear ratio is in it. There are no tags on front or rear axle to say what it is. Suggestions? I seriously think it is geared to high.
     
  2. May 26, 2023
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
    Jul 18, 2013
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    5,593
    It is always good to know the axle gear ratio.....you can take off the cover and look at the ring gear (tooth count should be stamped on it) or you can just count the teeth. You can also lift the wheel and count the number of driveshaft turns that occur with 1 or two turns of the tire in 3rd gear (1:1) on the transmission. Depending on whether it has a limited slip or is open diff, there is some variation in how you count the turns - a little google search will give you more direction than I can type. According to Jim Allen's book, 1977 cj OEM gearing was either 3.54 or 4.10. 4.10 would be pretty nice.
    Also, what size tire are you running? Taller tires are the same as increasing gearing. Guessing you probably have 3.54 stock gearing and some larger tires making it feel a bit sluggish?
     
    Last edited: May 26, 2023
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  3. May 26, 2023
    skipilot

    skipilot Member

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    Dec 16, 2007
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    That’s one of the tough things with a 3 speed. If you’re about 45 in top gear at 1800 rpm with 3.54 gears, I’d guess you’re running something like a 31x1.50 for tires. That would put you at about 2500 rpm at 65, which is not bad. You have to pick your poison, good low end or comfortable cruising. It all depends on what you want to do with your Jeep.
    Tim
     
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  4. May 27, 2023
    jeepdaddy2000

    jeepdaddy2000 Active Member

    Eagle Point oregon
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    Jun 24, 2004
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    1,132
    All common three speeds and virtually all 4 speeds have the same final drive ratio (1 to 1).

    Virtually a certainty.
    You need your tire size and gearing.
    Google a calculator and plug that information in.

    A good number is in the 2100RPM range at 60MPH. If your gearing/tire size drops the engine outside your desired RPM range, then you will need to change the tire size, gearing, or both.
    Lowering the diff gearing/smaller tires will lower all the transmission gear RPM's, which will effect first gear as well.
     
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  5. May 28, 2023
    colojeepguy

    colojeepguy Colorado Springs

    At the foot of...
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    A 3.73 gear ratio (common for an intermediate CJ) will give you 1880 rpm at 45 mph, with a 30" tall tire.
     
  6. May 28, 2023
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
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    Jul 18, 2013
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    might be helpful to post up more info on your jeep as well....tire size, does it still have the original transmission, axles, etc, or has it been messed with. Post up info with pics either here or in the intro section as well - the more we know about the jeep the better chance we can get you good information.
    The original equipment for 1977 would be a T150 transmission, D20 transfer case, D30 front axle and AMC model 20 rear axle, most likely with 3.54 gears. But some optional equipment would have been a T18 4 speed trans and 4.10 gears. One thing to keep in mind, 1800rpm is just about the peak torque rpm for the straight sixes, if you should have pretty good throttle response at 45mph at 1800 rpm in 3rd gear....
     
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  7. May 28, 2023
    AndyinTexas

    AndyinTexas Member

    central Texas
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    Jan 30, 2023
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    68
    No tag, but there should be a letter code stamped on the housing just right of the cover. For `77 the letter A would be for a 3:54 one-wheeler, the letter L for 4:10.

    If you find an AA you have the dreaded 2:73 or BB is 3:31 but you probably won't given your engine trans combo.
     
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  8. May 29, 2023
    Cornsipper

    Cornsipper New Member

    Memphis
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    May 26, 2023
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    I am running 265/70/16 so 30.1”x10.5. I can’t find any stamps with letters on the rear end. Original size tire was 29.9x9.6” so tire size is close except for width. When I bought it, it had 33” on it and damned near would take off unless half throttle. Changed to these tires and it made a ton of difference.

    I have another question. All I want to do is just putter around town and occasionally drive in country off road…not rock climb or anything extreme. I can run in low 4 for the off reading though. It does cruise fast, and is responsive in higher gears at 45mph and such. Just worried about it not having low end take off. I am going to go completely through drive train this winter and replace all bearings, races, seals and such. Also thought about maybe investing a little money in a new 304. If I decided to go with 304 would that mean I would have to change any of axles, transfer or transmission? Also the 4x4 stick has massive play and linkage is worn out so I am going to go with twin sticks while I have it in my shop.
     
  9. May 29, 2023
    dvader

    dvader Member

    Northern Nevada
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    Jul 20, 2006
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    Your axles are fine. I think you should consider a transmission upgrade. A 304 would work fine in front of your T150, but what it sounds like you need is a lower "start out" gear. Most truck transmissions, like the T18 do not have a syncronized 1st gear, so you would need to look at the 2nd gear ratio. Find one that is lower than the 1st in your three speed. For that, I would recomend the SM420 and NP435 in that order.
    An even better option might be a narrow ratio T19? Nice low syncronized first gear, a easy jump between gears, and a short enough length with adaptor that you may get away from drive shaft lengths mods?
    Check out the Novak website: https://www.novak-adapt.com/knowledge/transmissions/manual/t18-t19/
     
  10. May 30, 2023
    dnb71R2

    dnb71R2 SuperDave 2023 Sponsor

    Grand Mesa, CO
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    Remember one thing about the SM420...the newest model is still 50+ years old.
    The NP435 is a good choice along with the Ford T-18. The T-19 may be hard to find.
     
  11. May 30, 2023
    dvader

    dvader Member

    Northern Nevada
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    I agree, but the my problem with the T18 is only that the 2nd gear ratio (the start out gear) won't give him much advantage over the first gear ratio of the T150.
    I upgraded the tranny in 74CJ5 decades ago. I have a NP435 behind the stock 304. Later I put TeraLow gears in my Dana 20. My use of the Jeep has changed over the years. If I was to do it now, I would go the T19 route for the reasons stated above. I do see T19s now and then on Facebook Marketplace. The trick for me would be to find the correct T19.
     
    Last edited: May 30, 2023
  12. May 30, 2023
    Sierra Bum

    Sierra Bum Member

    The High Sierra
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    Oct 23, 2017
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    How about a T176 behind the new 304? Are those available? Fully synchronized 4 speed, strong enough for a V8, a bit lower first gear and fully compatible with AMC engines. Sounds like Cornsipper does not need a deep first gear… may as well be fully street-able.
     
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  13. May 30, 2023
    dvader

    dvader Member

    Northern Nevada
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    Yup, that would fit the bill for having a lower "start out gear." He would have to ditch his Dana 20 for a Dana 300. Not a bad choice either since he would have a lower low range and the added pleasure of helical cut gears in low range for a quiter ride!
     
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  14. Jun 6, 2023
    Keys5a

    Keys5a Sponsor

    Florida Keys
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    Speaking from experience, a T176/300 behind a 304 or 360 is a nice all-around combination, just maybe not for rock crawling.
    -Donny
     
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