Rear ratios

Discussion in 'Trucks [ Eighteen Wheelers ]' started by NFDDJS, Aug 24, 2012.

  1. allan5oh

    allan5oh Road Train Member

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    Being able to take off with 2.64's and a 18 speed shouldn't be a huge concern. The 18 speed has a 14.4:1 low gear. The math:

    14.4 X 2.64 = 38.016:1

    Direct drive 10 speeds are only 14.8:1 in first gear. The math:

    14.8 X 2.64 = 39.072:1

    Not a huge difference, how about regular setups? 13 speed and 3.36 gears:

    12.31 X 3.36 = 41.3616:1

    10 speed "b" gear and 3.55 gears:

    11.06 X 3.55 = 39.263:1

    18 speed and 3.36 gears:

    14.4 X 3.36 = 48.384

    So I would consider anywhere from 38:1 to 41:1 to be "normal" start gearing. Of course it depends on tire size as well.

    Cowpies setup is interesting. You are supposed to gain 2%-3% or so by running in direct. If it takes you 250 hp to go down the road, you've just found 5 to 7.5 free horsepower. But I think he might be giving up a bit with the 18 speed over a 10 speed direct. More moving parts = more loss. I think the 2-3% is assigned to the 10 speed direct compared to a traditional overdrive.
     
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  3. savage02

    savage02 Light Load Member

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    I'd think for that kind of weight a 18spd with a 3.70 or 3.91 would be pretty sweet.
     
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  4. allan5oh

    allan5oh Road Train Member

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    I'm assuming 11R24.5 tires. That's a very long legged setup for 80k-120k gross weight. It will be hard on the driveline. There is no magic setup here to get better mpg because you've got just about every factor working against you. Heavy weight, I'm assuming a "classic" truck, 24.5 tires, and 70+ mph. Most Canadians with that kind of weight run 4.11 or even 4.30 gears up here. What area of the country are you running?

    Here's some charts I made many moons ago:

    33613speed.jpg
    13speed355.jpg
    13speed373.jpg
    39013and18speed.jpg
     
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  5. allan5oh

    allan5oh Road Train Member

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    I think running fuel efficient 22.5LP tires with 3.73 gears at 63-65 mph would be a good compromise here. Engine RPMS would be 1340-1370. You would gain easily .7 MPG if not a full MPG over running whatever gear ratio with 11r24.5 tires at 70 mph. Plus it will be easier on the driveline and driver.
     
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  6. Cat sdp

    Cat sdp . .

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    18 speed, 3.73 rears and 24.5 tall rubber. Works good hauling heavy in the northeast but your FL runs not so much. Your looking at a compromise here!
     
  7. NFDDJS

    NFDDJS Light Load Member

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    From late spring till the end of fall we run from Ellsworth Maine most of the time to anyplace from Maryland to Florida depending on the market pricing. During the winter 90% of the runs we do we pick up from the Boston MA area, Portsmouth NH, or Portland ME to any where from water town NY and Colebrook NH.
     
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  8. Cowpie1

    Cowpie1 Road Train Member

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    Since the idea is to run in direct drive (16th) for max efficiency, then the 2.64's in 16th are no different than 3.55's in double overdrive. Exact same road speed and rpms. So it isn't any more long legged. Basically you are working one full gear lower with my setup. Other than that, no difference.
     
    Last edited: Aug 26, 2012
  9. Cowpie1

    Cowpie1 Road Train Member

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    No more moving parts in direct drive (16th) with an 18 compared to direct in a 10. If moving parts were a problem, then also tranny temps would be higher. But tranny temps on normal running average between 120F and 140F. The highest the tranny temp has gotten, working a gross load on the hills in 95F weather has been a shade above 150F. This is no different than the typical 10 spd direct tranny. When in direct, the input and output shafts are linked, so there is no other gearing involved. And an mpg avg of 7.5 this truck has gotten since I bought it is not something to necessarily sneeze at. The only real difference between running my 18 in direct and a direct 10 is that I have a lot higher torque rating than one can get in a 10, and I have all the splits I need to work just about any load. If it can comfortably pull a gross load across the hills of southern Iowa on hwy 2 or hwy 34, or highway 20 in Illinois from Dubuque to Stockton, which it has a few times, then I have no worries. For those that have never run hwy 20 in NW Illinois with a heavy load, you are in for a treat when you do. Especially at night.

    I have not had one time that startability was an issue. And I have taken gross loads to the house which the route is very hilly on narrow gravel roads. No easier running than what the grain and livestock haulers who run in my area have to do. And not even a pause running that with 2.64's.
     
    Last edited: Aug 26, 2012
  10. Cowpie1

    Cowpie1 Road Train Member

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    But you are assuming that one is always in double over (.73) in your charts. Why? What is the mindset that says one always has to use each and every gear available? Why couldn't one just use the first over as their top gear? Heck, we had single over 13's and 18's in the past. Just because they all come with double over's now doesn't mean we have to use double over. Make up some charts to show single over (.86) and how it would look. Then the 3.36's are not that big of a deal. Most do not use all the gears on the bottom, why would they always need to use all the gears on the top? I never use the top two gears in my setup, and have never had any desire to. But I have used every gear below that at one time or another.

    I use a road speed calculator spreadsheet that I can plug any tire rotation, tranny gear ratio, and rearend ratio in to, and it will give all the speeds from 1200 - 1600 rpm. I can change things on the fly and get new values. Using .86 hooked up to 3.36 rears on low pro rubber, the road speed at 1400 is 56 mph and 1600 is 65 mph. Direct drive is 49 mph at 1400 and 56 at 1600. Seems like plenty of room to work to me.
     
    Last edited: Aug 26, 2012
  11. allan5oh

    allan5oh Road Train Member

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    Yes actually there are. That's why there's more gears available. Power is still going directly through, but the other extra parts are still spinning.
     
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