Changing gear ratios

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Cdubb, May 22, 2022.

  1. Cdubb

    Cdubb Light Load Member

    Mar 22, 2015
    Hey all,

    I got a 95 International 9200 with a 375hp CAT 3176B, an Eaton RT1260A direct drive trans and 3.91 rears on 11-22.5 tires. Trying to get it more highway friendly, been playing with the Spicer ratio calculator trying to find something that might work a little better for me. Any suggestions on a gear ratio for me or would I be better off with a trans swap?
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  3. MadScientist

    MadScientist Light Load Member

    Dec 14, 2019
    I assume you mean Eaton RT12609A? A "1260" in Eaton's numbering system would have zero gears.

    How much weight do you typically gross with the loads you haul most of the time? What kind of routes do you usually run? Flat interstates? Non-interstates in mountains? Etc.

    How fast do want to cruise?

    Speccing a truck out is always about the application it will be used for. If you really want to know what would be best for your application, you need to tell us what it is.

    By the way, back in the mid-1990s I was assigned a new FLD112 for about a year with the 3176 in it. Best 10/11 L motor I've ever run. On hills with heavy loads it beat the crap out of the contemporary Cummins L10/M11/M11+ engines we had in most of our trucks.

    I once loaded 88 bales of cotton in Las Cruces, NM and climbed the hill from Alamogordo through Cloudcroft without ever dropping below 35 mph.

    It was nowhere near what a 14L (or 12.7L DD60) could do, though.
  4. Big Road Skateboard

    Big Road Skateboard Road Train Member

    May 2, 2021
    Are you pulling, or wanting to pull heavy losds? You're probably going to need those 3.91's if so.

    An RTLO 18 swap would definitely help you be able to get a bit more out of that truck, and give you the cruising speeds you're looking for.
    Cdubb Thanks this.
  5. Cdubb

    Cdubb Light Load Member

    Mar 22, 2015
    Im glad to hear you speak highly of the 3176, I know it has a mixed reputation but I think most of the negativity stems from the early ones or guys who expect the power a much larger engine has.

    I haul grain and feed so I run right at 80k when loaded, mostly flat lands, I’ll never see the mountains in that truck. Irish Hills in Michigan would be the steepest stuff it would see. I plan to run a lot of 55mph speed limit roads but I need the flexibility to also be able to run at a respectable highway speed, no less than 65 with out maxing my RPMs out. It worth noting my main elevator I work for has some city driving too which right now it’s not set up bad for that until I hit the highway. I have access to a van trailer so I would like to be able to pull loads from a board during lean times and around here those require the highway or turnpike. Right now 1900 rpm puts me right around 60-61mph, fastest I’ve gotten it on flat ground is 63 and she was screaming. I’ve only gotten a small sample size but I’m getting around 6.5-7 mpg mixed driving with idle time so I think this motor doesn’t mind RPM but I’ve gotten conflicting opinions about that. I’m thinking if I can get it down to 17-1800 rpm at 68 that would be ideal unless we should aim for lower RPMs than that.

    Yes I messed that up RT12609A is the current trans

    @Big Road Skateboard

    I would love a 13 or 18 speed again, ran an old KW with a B model and a 18 speed as a company driver, that old girl spoiled me. I know I’ve seen some 13 speeds that had a .87 or .86 final drive ratio, I’m sure 18s can have that ratio too.
    MadScientist Thanks this.
  6. AModelCat

    AModelCat Road Train Member

    Jul 7, 2015
    I'd swap the transmission to an overdrive model before I messed around with the diffs. Probably the cheaper route of the two.
    spsauerland and Cdubb Thank this.
  7. uncleal13

    uncleal13 Road Train Member

    Apr 9, 2009
    Humboldt, Sk
    I swapped out ring and pinions from 3.55 to 4.11 back in 2007 it cost $5,000 at that time.
    Cdubb Thanks this.
  8. bad-luck

    bad-luck Road Train Member

    Nov 16, 2013
    Baltimore, Maryland
    I think 355's will work for you. I wouldn't go lower than 342 though.
    Cdubb Thanks this.
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