Here's what I have so far.

Discussion in 'Ask An Owner Operator' started by LAD_Mavric, Feb 18, 2008.

  1. LAD_Mavric

    LAD_Mavric Light Load Member

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    Feb 3, 2007
    Midland, ON
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    This isn't soo much a question as it is an update as many of you have had input talking about spec's for my ProStar in other posts, Here's what I've come up with so far; (Let me know if you see any major problems or I'm way off base with something)

    -'08 ProStar with side skirt package
    -Cummins ISX 500
    -13spd autoshift
    -22.5 tires (possibly X-One's)
    -3.60 rears

    Also I'll be getting a headache rack as I will be pulling a flatbed most of the time. I know you don't like the '08 trucks very much but I'm willing to gamble with that for now. All my research leads me to believe that I should be able to get 6+ mpg out of this truck most of the time.
    Let me know what you think.
     
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  3. MedicineMan

    MedicineMan Road Train Member

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    I'd go for a higher rear end personally. I like 3.55 max, 3.33 better(and that's with tall 24.5 rubber). But I like to run where the speed limits are 70 and 75. If you run slower most the time then that looks like a good setup. I'd get an extended motor warrentee if I were you. I havn't heard of as many problems with the new cummins as the cat but I know they are still having some.
     
  4. LAD_Mavric

    LAD_Mavric Light Load Member

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    Feb 3, 2007
    Midland, ON
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    Yeah I was looking at getting a 3.33 or 3.4 rear but here in the wonderfull province of Ontario they are trying to pass a law that would have all the trucks that run in the province forced to have speed limiters!:biggrin_25516: They want us to be limited to 105 km/hr (roughly 65mph) so that being the case I'm looking to spec for that speed. I don't want to spec a truck to run at 70mph and get screwed when / if I get limited to less than that. Plus I'm not a huge speeder anyway.
     
  5. MedicineMan

    MedicineMan Road Train Member

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    find a gear ratio calculator. I think eaton has one on there site somewhere. You want to spec the rear end for a gear that will keep your motor at it's sweet spot at 65 then.
    What's the sweet spot on an ISX guys?? 1400? I'v never driven them but I think they are higher than the cats.
     
  6. LAD_Mavric

    LAD_Mavric Light Load Member

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    Feb 3, 2007
    Midland, ON
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    Yeah I'm in a company truck right now with an ISX 400 and it is right at 1400rpm but that's in a 2000. It would be a bit lower on the newer motors right?
     
  7. MedicineMan

    MedicineMan Road Train Member

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    not necasarily. its the same motor basically. I wouldn't want to be cruising over 1450. I'm down at about 1350 at 70mph. That's more a guess though from other trucks I'v driven with the same setup. My speedo and tach are still broken
     
  8. Eskimo6804

    Eskimo6804 Heavy Load Member

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    Northeast Alabama
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    Sweet spot on the egr motors(2004-2007)is 1250-1350. I'm not sure about the new '08 junk though. To run in that rpm range with 22.5 tires and an overdrive tranny, you want 3.55 or 3.58 gears(1325 at 65 mph). 3.42 will put you on the low end of it(about 1250 rpm at 66 mph).
     
  9. LAD_Mavric

    LAD_Mavric Light Load Member

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    Midland, ON
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    Ok that then brings me to my next question. Will a 13spd automatic hold a gear at that low an rpm or will it want to drop one?
     
  10. 7mouths2feed

    7mouths2feed "Family Man"

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    May I ask where you got these calculations.... Looking at used trucks and that formula would be usful.. Also where can I find sweet-spot (assuming max torgue is the sweet spot) for older engines. Thanks
     
  11. Eskimo6804

    Eskimo6804 Heavy Load Member

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    When you have spec'd as many new trucks as I have, you kind of get a feel for it. Most of the info that I post here comes from the experience I've gained from running my operation, and this one is no different. There is a calculator to figure what rpm you would run based on your specs, but the only way that I know to access it is through the sales dept. at your local truck dealer. All of the dealerships use a formula which you can enter all of the variables(gear,tire size,tranny,motor)and it spits out the speed/rpm matrix that they use when someone orders a new truck.

    On the pre-egr motors(1999-2003), you can roughly add about 100 rpm to the previous number and that is the general range of the sweet spot for the three major mass produced motors(about 1325-1450). The true key to spec'ing any truck is; make the specs match your driving habits and not to try to match your driving habits to the specs. That's why LAD is going about it the correct way. He decided what cruise speed he wants to run at and he is spec'ing the truck to run most efficiently at that speed.
     
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