Discussion in 'Start Your Journey Here' started by endura, Sep 11, 2007.

  1. warthog1984

    warthog1984 Well-Known Member

    The fuel usage for a vehicle is not linear (straight). There is a threshold to maintain operation (600-1000 rpm depending on vehicle) that just keeps the ICE and powertrain running. This is "wasted" rpms/fuel.

    Beyond that, fuel usage goes up linearly with rpm. However, POWER (not rpm) is what supplies the energy to overcome friction and aerodynamic drag. However, the available Power to the wheels does not go up linearly. It varies according to the "Torque Curve" because of the nature of an ICE. Thus, fuel usage and speed are not proportional.

    Also, aero drag is a function of the SQUARE of the Speed, so it takes more power at higher speed to maintain cruise.

    Thus, every vehicle has a series of overlapping slightly misshaped bell curves that determine the most efficient speed at each gear.

    The most efficient point is each gear is the point with the lowest RPM/Speed qoutient for each gear. The most efficient overall speed is the speed with the lowest RPM/Speed quotient period. Generally one of your top 2 gears.

    Going back to my truck, it turns at 1600 rpm at 55 and 1800 at 65, which means it is 5% more efficient per MILE. At 75, it turns 2300, which is 5% less efficient than 55 and 6% or 7% less efficient than 65. At 45, it turns 1325, which is less efficient than 55 but saves the acceleration gas.

    Hope that helps.
  2. psic

    psic Well-Known Member

    I'm a bit confused. RPM/Speed is a linear function, how does torque come in? I mean to say, if I calculate RPM/Speed for my car in fifth gear, I'll get .25 for every combination starting with 1500 RPM and going up. I'm guessing that it's the same in the other direction (lower RPMs). So how do I find the most FE friendly speed in fifth?
  3. benffv

    benffv Well-Known Member

    swap that gas truck for diesel. the ford f250/350 4x2 diesel gets around 17mpg and maybe 15mpg fully loaded( can easily handle the 2000lbs vs the struggling 5.4L)
  4. warthog1984

    warthog1984 Well-Known Member

    Most efficient speed is influenced by engine, gearing, aerodynamics, rolling resistance (friction), mechanical friction and several other factors. The Only way to find the most FE Friendly speed is to take a Scangauge out and find the highest FE or do a test run noting the rpms and speed for your car and doing the rpm/speed calc.

    In an ideal world, it would be a simple theory calc, but in reality, other factors heavily influence FE. That is why hypermiling is better for FE than steady-state driving: a car on the roadway is not a theoretically mechanically perfect machine and it experiences heavy energy losses.
  5. desdemona

    desdemona Well-Known Member


    I have been reading this thread, no nothing about trucks, but.. I hope you do go ahead and try and get the best mpg possible to get out of the Beast (you can have that name-- I had a car that I named that once). Anyway, I don't know if it sounds like for economic reasons you can just sell the thing and get something else. However, I am hoping to see your mpg figures and get that "skilled hypermiler" by your name, even if it only means you are getting 17 mpg or less.

    I do think a Scan gauge will help a lot. I have already paid $25 on it for my first month (that much saved in gas).

  6. endura

    endura Member

    I researched a diesel but it's not cost effective whatsoever for my application. Loaded yes, they are very impressive vs gas motor, but for my application, which is really a light load, their benefits are very small in real world applications.

    If they come out with a smaller diesel motor for a 1500 series truck, I'll be the first in line to get one.
  7. endura

    endura Member

    so on a steady state cruise, all other factors being equal, which of the above examples nets you the highest MPG?

    what are your MPG numbers for those speeds?

    it's a dakota, right? what's the rear end gearing?

    Last edited: Sep 12, 2007
  8. warthog1984

    warthog1984 Well-Known Member

    Steady state the 60-65 mph band is the best (lowest rpm/speed at 27.7). Exact speed depends on washed truck, drafting, etc...

    Normal driving I'm getting 15.5/18 for 50/65 mph. Paying attention to FE, 17/22 without much effort (watching acceleration, A/C use).

    Hauling rebar in the field, 15 or 16 mpg from the office into the field (offroad) and back.

    If I Pump up the extra shocks I put in, I can haul a long ton and get 13 or 14 MPG on a 50/50 city/highway route. Don't drive it for long like that because its supposed to be a half-ton and braking takes a while.

    Stock gearing, I'll check what it is. Tires are slightly oversized for offroad use.

    Yeah its a Dakota Club Cab. Yeah its got a 318 (5.2L) in it. Yes, I can smoke the tires without double footing. :D

    If that truck could swap engines for daily driving I'd never let it go.
  9. warthog1984

    warthog1984 Well-Known Member

    BTW- if you can drive steady state and note the rpm at 5mph intervals, I can tell you which is most efficient.
  10. endura

    endura Member

    1200 38
    1300 41
    1400 44
    1500 47
    1600 51
    1700 54
    1800 57
    1900 60
    2000 63
    2100 66
    2200 70
    2300 73
  11. warthog1984

    warthog1984 Well-Known Member

    How many gears do you have? Are these observed rpms or theoretical? Usually this would cover 3 gears and a range of mechanical and friction losses. Remember, there will be slippage and friction losses in a truck.

    Remember, this is a rear gearing and a reduction gear in the tranny Also.
  12. endura

    endura Member

    this is top gear RPM/SPEED. with the TQ locked. I calculated off of tire diameter/rear end ratio/top gear ratio. observed results off speedo/tach. match exactly.

    example. at 60mph, I am turning exactly 1900RPM.

    What does slippage and friction losses have to do with this. this is top gear(4th) with converter locked up.
  13. warthog1984

    warthog1984 Well-Known Member

    Your engine rpm/vehicle speed results are linear across a 35mph range. This is physically impossible. Power transmission from the cylinders (rpm measured) to wheels (speed) is imperfect. These results violate Newton's 2nd law (KE=1/2 mv^2), Bernoulli's theorem (drag is prop. to v^2), and the nature of an ICE engine (combustion energy vs power as inertia changes and engine overspeeds flame front).

    Not to mention, if this is fourth gear, what are you doing with your other 3 gears? Usually each gear covers 20-25 mph. IE, 1st 0-25; 2nd 25-45;3rd 40-55;4th 55+. If this is correct, You're probably running horribly deep gearing. No wonder your gas mileage is bad. What do you do, tow semis for a living?
  14. ILAveo

    ILAveo Well-Known Member

    Nah, since he's got the heavy duty package, I bet he has a granny first gear that doesn't get used much that is provided in case he wants to tow a backhoe around. For purposes of light duty hauling it's probably about like having a three speed.

    Before you start worrying about Newton's law vis-a-vis RPM/MPH you have to realize that you can have different torque/energy output for the same RPM-MPH combination. Think about your torque and throttle position for steady-state uphill vs steady-state downhill at the same speed. It takes a lot more torque and throttle up-hill vs. down-hill. I think you over-simplified your analysis someplace.
  15. endura

    endura Member

    ok, I'm officially confused.
  16. psic

    psic Well-Known Member

    I get similar results, that is to say rpm/speed is linear across the entire range (up to 3000 RPM anyway, which is as high as I go) in fifth and fourth gear. I think this should be normal, except for any losses between engine and wheel (by losses I mean inefficiencies, which are probably not linear). These are relatively small and account for the error when reading RPM and speed from the dash... at least, that's how I see it.

    If you take out inefficiencies (like the loss you get whenever you transfer the motion, in the gearbox for instance...), then RPM/Speed IS linear.
  17. warthog1984

    warthog1984 Well-Known Member

    Well I was thinking this was all on flat ground with slow, steady acceleration.. I wasn't going to get into uphill vs. engine braking and all that jazz.

    Still, having only the one gear for that big a range can't be the most efficient. Using OD would probably save some gas at the top end. Why are the rest of the gears so deep? I can buy a granny gear, but can't see why the 2nd and 3rd would be so deep. And again, Where's the OD? I haven't seen a 4speed auto on a truck yet.
  18. endura

    endura Member

    I thought you were talking what my most efficient speed is at a steady state cruise, top gear with the torque converter locked. that is the graph I gave you.

    on my truck, I'm assuming that cruising at 1200rpm/mph is my most efficient cruising speed since below that rpm the torque converter hunts quite a bit and won't consistently stay locked.

    so, is my theory relatively sound? the most efficient STEADY STATE cruising rpm on a modern non-carburated vehicle is the lowest point in engine speed that the vehicle can stay in top gear with the TQ locked consistently.

    I did order a scangauge, BTW.
  19. endura

    endura Member

    it's a 4speed automatic, and yes, you are absolutely right. with the mandatory 4.1 rear end gears this truck comes with, 4th gear is way too high for efficient cruising. it's insane to have a 5.4L V8 with a very wide and flat 365lb/ft torque curve turning almost 2000 rpm at 62.5mph. this engine can pull this truck at that speed, even with minor grades, at 1400/1500rpm without any problems. I can understand for someone who tows large loads they would need these gears, but for my application, the fact that I need a 4 door with an 8foot box pigeonholed me into a truck that is set up from the factory, no choice, to tow 10000lbs of weight.
  20. warthog1984

    warthog1984 Well-Known Member

    This might be a case where shelling out the $ to have someone drop and regear the tranny would be worth it. If you have a flat torque curve and truly don't need the full load/tow capacity, switching to taller gears (include an OD gear )might save big bucks on gas over 2 or 3 years. That would leave a granny gear, low speed, medium speed, and OD and still leave load capacity. Just be warned that pulling the tranny runs about $1000-$15000.

    Also, for your truck, 35-40 depending on idle might be the best right now. Let me know what the SG says.

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