swap in a 13?

Discussion in 'Trucks [ Eighteen Wheelers ]' started by rank, Mar 26, 2015.

  1. rank

    rank Road Train Member

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    The truck in the Sig has a deep reduction 15 single over, 3406b, 3.90 rear, 24.5 LP. I usually cruise ~60-65 @ ~1600-1700 rpm hauling 80,000 in the Appalachians. Getting 5 usmpg on winter blend and 6 summer.

    I'm not too fond of the big gaps between gears and the higher cruise rpm compared to the 13 double over. Will a 13 save enough on fuel to pay for the swap?

    The trans would cost $5000 exchange + labor. The clutch doesn't need doing.
     
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  3. double yellow

    double yellow Road Train Member

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    If I'm understanding correctly, you'd be going from a .79 top gear to a .73 top gear. That would lower your rpm range from 1600-1700 to 1478-1571.


    Now, let's assume you have a Detroit 12.7 (because I haven't seen a BSFC chart for the CAT). And let's assume that you're averaging 200 horsepower.
    View attachment 81257

    At 1650rpm and 200hp, the BSFC is ~0.324. At 1525rpm and 200hp, the BSFC is ~0.320 -- a thermodynamic efficiency improvement of 1.2%


    So instead of 5.5mpg you'd get 5.57mpg. At 100,000 miles/year and $3/gal in fuel, you'd save ~$650/year in fuel meaning you'd be looking at a ~10 year payoff.


    Zero real-world experience, by the way, that's just what the physics tells me the difference is in top gear. You may gain additional mpg from not having as big of a drop between gears too.
     
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  4. Cat sdp

    Cat sdp . .

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    If you don't need a clutch/ trans or the rear main seal done I'd save your money . And enjoy those low gears when needed.....
     
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  5. BigCam9670

    BigCam9670 Medium Load Member

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    I have a rtlo16713a with 3.90s and a 3406e and run the same area as you and cruise right around 62-65 most of the time, 1450-1500 rpm on tall 24.5s. low gear is plenty low for what I do and I see a lot of jobsites...it works well for me. When this transmission goes, I will swap an 18 speed in to have the best of both worlds, I get 6.5 winter and 6.8 summer with my setup. Dropping your cruising RPM to 1450 will make a world of difference. That is a cool truck BTW!
     
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  6. rank

    rank Road Train Member

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    Thanks.

    Interesting comparison. I got 6.0 to 6.3 last summer. I wonder how much of that is the gearing and how much is the E model. Are you usually 80,000?
     
  7. Hammer166

    Hammer166 Crusty Information Officer

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    Edit: Brain Fart
     
    Last edited: Mar 27, 2015
  8. BigCam9670

    BigCam9670 Medium Load Member

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    We went from 15 to and 18 and no modifications to the driveline were required. I imagine a 13 would be no different.
     
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  9. BigCam9670

    BigCam9670 Medium Load Member

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    We also have a 93 379 ext. with a 3406C fully mechanical, an rtlo18918b and 3.70s and that truck does well over 6.5mpg doing the same type of work and similar cruise speeds. In my opinion the gain in fuel economy and the ease of driveability of the the 13 speed will be well worth the expense. How much extra is the 18? Might be worth the extra $$ to have the low low gears available if needed.
     
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  10. Hammer166

    Hammer166 Crusty Information Officer

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    Yeah, I had it wrong in my head, the 10's are the shorter ones.
     
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  11. BigCam9670

    BigCam9670 Medium Load Member

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    yeah 10's are no doubt shorter
     
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