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Thread: Rear ratios

  1. #11
    Road Train Member sdaniel's Avatar
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    The O/P plans of running standard 24.5 , with 3.36 gears and tall rubber . He will be very close to cowpies "final" drive ratio . Cow is running lopro 22.5 . With the higher rolling rest. it may be a tad worse ? Just one aspect appears to be over looked. Sni , walmart , usa , mavrick have all been using the 2.64 direct path several years . If its holding up under green horns at 80k . It should do well with us at our skill levels. Heck we ran 7 speeds , no back box robbing 3 to 5 percent of your hp . That was years ago on a ded gov run . Felt good getting over 10 mpg in 86 in one of these loaded at any weight !


  2. #12
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    Quote Originally Posted by Cowpie1 View Post
    I have a Columbia glider with a 500 Detroit DDEC IV 60 and an 18 spd and 2.64 rears and have no problem starting off with a 80K load or running two lanes with hills with that same load. I don't lose gears any faster or more on any hills I run compared to a traditional 3.55 rear. I am using Meritor 14x 40K rear axles. I realize this setup is not for everyone, and I wouldn't recommend it for running over 80K, but it works pretty well for 80K or below. And running in direct 16th at road speed, it is averaging 7.52 mpg. Worse was 6.7 pulling near 80K up and down two lane, hilly roads for the majority of that run. And tranny never gets above 150F and usually between 125F and 140F. Same for axles. And engine EGT's never above 900F on even the hardest pulls. I pull right along with others running similar weights and using shorter rear end ratios.

    The key was to run, at normal road speeds, in direct drive (16th) for more efficiency. The senior engineer at Eaton, Bruce Malinson at Pitts Power, and others have shown that there is substantial power loss by running in overdrives. Eaton has shown that there is a 3% power loss to the rear wheels in overdrive, and Malison has stated (not sure if his numbers are totally accurate) that there is a loss of over 40 hp to the rears when running in overdrive. I can confirm that following these guys, and others, advice, my setup is doing great and the mpg numbers are better than most. I took a shot and it worked. Sometimes it does pay to think outside the box.
    cowpie is right. the high ratio of 3.36 and those 2.64 is fine if your grossing 80 or less over the road. i still dont like that high of a ratio, it is to stressful on the drive line (from the clutch to the rearend). if he is going to gross 120000 like he said 3.55 or lower would be alot less stressful on the whole drive line. i run 3.70s grossing up to 144000 plus and thats to high of a ratio for me it is easy to snap a u-joint or twist a drive line or shatter a rear in soft ground if you dont pay attention. my ideal ratio for what i do would be 4.10 but i would suffer a little bit of mpg. when i bought the truck that i drive now it had 3.36 in it pulled them to reratio them and the wear on the parts was very noticeable. the 3.36s were introduced for low rpm engines. if i remember right they wont even warrenty them for very long if you have high hp engine.

  3. #13
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    What has to be understood is that once you are moving it is the overall ratio that determines grade ability.
    Transmission ratio X Diff Ratio X tire revolutions per mile.
    If you have enough gears with small enough steps you will have similar overall ratios available than with a slower rear set up.
    You therefore will have no issues on a grade.
    The downside is that on pull away or while backing under a trailer you wont have the very slow crawl that can be useful.
    The upside is that you can run at cruising speed in direct drive in the transmission thus saving fuel due to less losses through the transmission.
    When bob tailing or empty you can high idle at road speed saving a lot of fuel in overdrive.

  4. The Following User Says Thank You to king Q For This Useful Post:


  5. #14
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    Quote Originally Posted by king Q View Post
    What has to be understood is that once you are moving it is the overall ratio that determines grade ability.
    Transmission ratio X Diff Ratio X tire revolutions per mile.
    If you have enough gears with small enough steps you will have similar overall ratios available than with a slower rear set up.
    You therefore will have no issues on a grade.
    The downside is that on pull away or while backing under a trailer you wont have the very slow crawl that can be useful.
    The upside is that you can run at cruising speed in direct drive in the transmission thus saving fuel due to less losses through the transmission.
    When bob tailing or empty you can high idle at road speed saving a lot of fuel in overdrive.
    i agree but the stress is still higher with a high ratio. me and stress dont get along very well.

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    I was looking at getting 3.58 rears before but now I am questioning if that would be to tall of a gear. I am looking to be able to run 70mph and be at a good over all RPM. With 3.58 rear ratio that will put me at 1500RPM when doing 72.3MPH and at 1400RPM I will be running 67.5MPH

    I got this all from that site below. I just know how I drive and it is 5-8 mph over the speed limit most of the time 70% of the driving will be in 65 MPH areas and the other 30% in 55MPH and 35MPH areas.

    I just feel that running 1400-1500 RPM most of the time with a C-15 is not going to be the best on fuel.
    A ratio of 3.36 might be better putting me at 71.9MPH at 1400RPM

    http://www.roadranger.com/ecm/groups.../ct_062746.swf

  7. #16
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    Quote Originally Posted by NFDDJS View Post
    I was looking at getting 3.58 rears before but now I am questioning if that would be to tall of a gear. I am looking to be able to run 70mph and be at a good over all RPM. With 3.58 rear ratio that will put me at 1500RPM when doing 72.3MPH and at 1400RPM I will be running 67.5MPH

    I got this all from that site below. I just know how I drive and it is 5-8 mph over the speed limit most of the time 70% of the driving will be in 65 MPH areas and the other 30% in 55MPH and 35MPH areas.

    I just feel that running 1400-1500 RPM most of the time with a C-15 is not going to be the best on fuel.
    A ratio of 3.36 might be better putting me at 71.9MPH at 1400RPM

    http://www.roadranger.com/ecm/groups.../ct_062746.swf
    like we all said 3.36 probably will be fine with regular freight. but if this is a single turbo cat and not a junk acert the best rpm IS around 1500 rpm for performance. i get my best mpg at 1550 rpm. but mine isnt your typical cat. when it was an acert the best rpm was 1350 rpm. there is alot of variables to consider with specs on a truck not just part of it. you say it will be running at 1500 at 72.3 so at 1450 you will be around 70 mph, right? i think you have your answer 3.58 would be the way to go. if 70 isnt fast enough then i dont know what else to say.

  8. #17
    Trucker Forum STAFF Oscar the KW's Avatar
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    Cowpie, someday (hopefully a long time from now) some mechanic is going to tear that trans apart and say holy crap driver, you ever had this thing in the big hole?

  9. #18
    Medium Load Member Don2WS's Avatar
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    Two of my uncles have 2.64 and 10 speeds on the ex walmart trucks are great on fuel mileage but anything over 40k and you feel the stress that you're putting on the drivetrain we try to stay less than 30k on these trucks. We also have a t2000 with 3.36 13 speed c15 acert and super singles and even with XDN tires the truck struggles to start off in the snow because it always hauls very light loads. The rest of the trucks are either 3.55 or 3.70 and they're either single turbo c15, 3406e, c16, and the cabover with a big cam, all have 13 or 18 speed. In my opinion 3.55 is the best choice because we haul dry van, and once in a blue moon flat

  10. #19
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    Dang. All this talk about 2.xx and 3.xx rear gears is making me sick. I've got 4.17's and if I changed 'em, I'd go to either a 4.35 or 4.42 ratio. I've driven trucks with 3.36 and 3.55 gears before, and they couldn't pull themselves up a hill...funny thing is, I got the same fuel mileage out of them as I do from my current rig....but my current rig drives a heck of a lot better.

  11. #20
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    Right with you Pedigreed Bulldog. I have posted and argued many times before on this. I will burn alittle extra fuel anytime if it saves some wear and tear on drivetrain. Rarely under 80K and venture off road almost daily, the lower rear gears make a big difference. Always good knowing when i let the clutch out the truck is going to idle out of whatever hole it is in, without broken parts.

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