If U were spec'ng a truck for fuel mileage...

Discussion in 'Trucks [ Eighteen Wheelers ]' started by jagerbomber3.0, Sep 26, 2009.

  1. 25(2)+2

    25(2)+2 Trucker Forum STAFF Staff Member

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    Isn't it funny how educated idiots and people who can't drive tell us more about how to do it than people that might really know. Some companies still go with 13s, my small company is getting some back to replace what were sold off by previous owners.

    If you run all Interstate with no mountains at a certain speed with the proper ratio, they are probably right, but the best trucks I ever drove were 13s, even a 9 is a better choice than a ten because the lower ratios are spread more giving you more gears to run at the top end depending on conditions, better for pulling hills IMO.

    I drove 9s the first 150k or so, I dislike the straight 10 almost as much as 10 speed autoshifts. Put about 200k on several straight 10s; got an autoshift now, cheapest trucks I have ever been in with bad seats. Jobs are hard to come by, at least good jobs, not that this gig could be considered good by most.
     
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  3. Eskimo6804

    Eskimo6804 Heavy Load Member

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    Why would it run any hotter in 16th than it would in 18th? It's a serious question, I don't know. Educated me/us.
     
  4. 25(2)+2

    25(2)+2 Trucker Forum STAFF Staff Member

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    All the gear sets are churning that oil and are turning faster in the lower gears.

    Full synthetic cuts it some but the faster the gears are turning, the more heat that is generated,

    That can be countered by using the logic that the tail shaft turns more slowly with tall gears. I used to run a tall geared 13 speed direct in 12th a lot pulling the hard pulling trailers, and got better performance including mileage.

    Another argument for a 13 on even just midwest runs is wind affecting drag on the hard pulling trailers, you just can't pull it in high and sometimes not even 9th.

    I could make better time with that 13 even though it was governed out for speed in the top 3 gears at 68 mph, than I could with a newer truck and more horsepower with a 10 speed that would do an honest 76 mph with worn tires, that was clocked on an interstate with a stopwatch over 76 miles between mileposts. The speedometer read 75 and I was still being passed by trucks pulling reefers. That trip there was no wind and the roads dry, any slowing is harder to pick back up with a 10.
     
  5. 550hpW900L

    550hpW900L Road Train Member

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    You got your answer skimo. Gears turn faster, more friction, more heat. For instance if I'm rolling through ca or il doing 60mph in 17th my trans temp is right at 190. 72mph in 18th it's 150ish.
     
  6. stepnfetchit

    stepnfetchit Medium Load Member

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    I guess I'm really old school here. I had a 1996 379 ExHd 550 Cat with 3:55 and a 15 over. Owned the truck for 4 years. Kept good records and the co I was leased to and my wife helped me keep up with the fuel mileage. That truck averaged 5.83 mpg for every mile I drove it, roughly 412,000. I never really cared about how fast the truck would go, but I want a truck to PULL. I hate down shifting. That truck would pull 44800# up Monteagle at 53-54 mph. I know, somebody's gonna dispute it, but it was my truck I was the only one who drove it, so it was set up my way. I know. Truck speed up Monteagle is 45. Still believe the more gears you have to drop the worse the fuel mileage. 5.83 mpg is not bad for a Cat. The co I was leased to had some KW T2000 with 475 Cats, 13 speeds and 3:36 rears and my fuel mileage average was as good or better then theirs. I also had 1850# of torque. Remember HP gets you down the road,Torque gets you up the hills.I would spec a new truck out the same way. Aw can't do that.No Cats available now I guess. Besides, who wants a 15 speed today. Too much work shifting.
     
  7. 550hpW900L

    550hpW900L Road Train Member

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    I would take a 3 range 15 speed over anything ;)
     
  8. Eskimo6804

    Eskimo6804 Heavy Load Member

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    Would that mean that the 10 speed direct trannys use a different type of shaft or shaft system to overcome this issue? 10th gear in that tranny is a 1:1 ratio same as 16th in the 18 speed.

    It's no secret that I'm not a mechanic, but the inner mechanical workings of a transmission is completely greek to me.
     
  9. LoneCowboy

    LoneCowboy Road Train Member

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    WE need more info, there are so many variables.
    What are you going to pull?
    Where are you going to run?
    What type of trailers?

    And it's a complex question, it's not just gears and engine.

    The biggest mileage gains are by the driver.

    Aero trucks make a huge difference. trailer fairings, tank fairings, etc. Long hood conventionals don't do so hot.

    22.5's get better mileage than 24.5's
    Having one set of tires and a different gear set totally changes the effective rear axle ratio.

    More gears can help, but also doing all that churning doesn't help. Direct drive rather than overdrive does do better.

    Super singles do better. Heck tread pattern alone makes a difference. Running single ribs in the summer will gain you 2% or so over winter tires.

    Pre-03's do a good 20% better than post 03's. (in the same truck, same driver)

    Lower HP motors get better mileage (to a point, you can't put a 300hp in a truck doing hills and expect it to do well), but putting in a 600hp motor will pretty much limit any gains.


    Again, we need more info
    What are you going to run and where? (generally)

    And, what's your budget?

    Then, we can spend your money. :biggrin_25521:
     
  10. Displaced Yooper

    Displaced Yooper Light Load Member

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    I know this is an old thread, but I just gotta comment on this.

    Running a 13 speed in 11th, or an 18 speed in 16th WILL NOT make it run hot. If anything, it will run a bit cooler than in the big hole. 11th/16th is a direct 1:1 coupling of the input and output shafts, without any gear reduction (multiplication in the higher gears, for the overdrive).
     
  11. Jfaulk99

    Jfaulk99 Road Train Member

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    I've ran a 13 in the lower gears for years, mainly because Ohio was a 55mph state and you couldn't run 13th unless you were over 65mph. I have never had a transmission get hot because of it.
     
    russellkanning Thanks this.
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