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Fuel weight affect on mpg?

Discussion in '6.9L IH & 7.3L IDI Diesels' started by Cubey, Mar 13, 2019.

  1. Cubey

    Cubey Full Access Member

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    Does fuel weight of carrying 2 tanks affect mpg much on IDIs?

    With my motorhome, I want to stretch every penny I can on fuel. My fuel tank selector switch seems to be working, so I'm debating if I want to use both tanks so I can fill up at the cheapest stations that may be many hundreds of miles apart. The front tank is probably an 18 gallon by how much it takes at the 1/2 tank mark, so at let's say 10mpg, I'd have to refuel after about 120 miles, even if prices are higher than they are another 100 miles down the road. If I have my second tank full, I can keep switch tanks and wait until the next cheapest station. I use GasBuddy so I try to chase down the lowest fuel prices that way, not by luck.

    But, is it cost effective mpg wise? If it's just a few pennies difference in price, it probably doesn't make sense. But sometimes it's 10-15 cents difference. For 10-12 gallons, that's $1-$2.25 difference. It adds up fast. Especially if I'm about to leave one state where it's 25 cents cheaper than the next.
     
  2. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    Well diesel is 6.943lbs per gallon, times 18 gallons= almost 125lbs.

    What does your motorhome weigh? My pickup weighs about 6600lbs, so that much fuel will *at the worst* represent 1.89% decrease in fuel consumption on my rig. On yours I suspect it to be almost half of that. On flat ground I suspect it would not change it a bit.
    Your ignition timing, right foot, and coefficient of drag have far far far more say in your MPGs than fuel weight.
     
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  3. Cubey

    Cubey Full Access Member

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    I don't know but here is the door tag.

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  4. Shadetreemechanic

    Shadetreemechanic Full Access Member

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    Your speed will have a much larger effect on your mileage than the weight of the fuel ever will. Fill up when cheap and keep it under 55. I have long since forgotten the formula for wind speed drag, but if memory serves me at all I believe drag moves up exponentially with wind speed.
     
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  5. Cubey

    Cubey Full Access Member

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    I stay at 55-60 generally, maybe 65 once in a while. I care more about MPG than getting somewhere fast, most of the time.

    My 6.9 NA C6 3.55 F250 pulling a 27ft travel trailer could barely stay at 50-55 in high wind and I'd have it floored.

    This motorhome is 4.10, unfortunately for me. It was set up for towing a car behind I'm guessing, by the ancient brake controller still in here. It could have been a cargo trailer though, who knows. But that's why the original owner sprang for ATS turbo. Can you imagine a 6.9 NA Class C motorhome with something in tow too?
     
  6. Shadetreemechanic

    Shadetreemechanic Full Access Member

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  7. jayro88

    jayro88 Full Access Member

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    I highly doubt you would see ANY difference in regards to weight from fuel. Your biggest enemy is wind resistance, not weight. I have towed a bit with my E250. It is a 7.3NA c6 with 3.54’s and a DNE o/d. The difference between towing a flatbed car hauler and a TT that weigh the same is HUGE due to the frontal area of the TT.

    I did an experiment with cruising speed unloaded. The difference in my MPG between setting the cruise at 60mph vs 75mph was almost a 20% increase in efficiency.

    If you want better MPG then here are the things you can do:
    1. Slow down. Remember these were designed when the national speed limit was 55mph.
    2. Maintenance! With a fresh fuel system I could get 19-20mpg hwy and 16-17mpg mixed was normal. With my now worn our system I can get 16mpg hwy and 12-14mpg mixed.
    3. Get an O/D, swap in an e4od or do both. I added my DNE2 o/d and it did a lot. It only raised my max mpg by 1-2mpg, but it raised my average mpg by the same amount. Plus, if I drove faster my mpg would only drop 1 mpg instead of 3 mpg. Much more forgiving. As a bonus I can now hear myself think.
    4. Swap out your stock c6 torque converter with diesel towing one with a lower stall. The stock one is way too loose for diesel rpms.
    5. Look into aero mods. Build a front air dam etc. There are tons of ideas out there.

    If you have the $ to invest I would recommend swapping out the c6 to an e4od and consider adding an aux overdrive unit. With the turbo you have some extra grunt. The e4od will give you overdrive and a locking torque converter.

    The real question is how much do u want to spend?


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  8. Cubey

    Cubey Full Access Member

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    There is an air dam actually. It fell off but it looks like I can remount it, despite the damaged fiberglass at the original mounting points at the ends.

    20190316_183619.jpg

    55-60 is my preferred speed, so no real problem there.

    The rest? Dunno about, I will have to see in time.

    It has new (cheap) LT rated highway tires, all matched, inflated to 65 psi, per spec of the motorhome company's door tag.

    I haven't taken it out on a long highway drive since it got new tires (it had rotten, mismatched tires of 3 or 4 kinds at least), and it had a nasty clogged k&n air filter and the air dam wasn't on it. I got about 8-9mpg highway with those issues.

    With new tires and a clean paper air filter (a standby filter), I'm getting 7 city with tons of stop and go. So I am hoping to see at least 11mpg hwy once the air dam is put back too.

    My truck, with AT tires and a tall campershell , gets about 10 city, 14-15hwy with a fair amount of oil consumption. I'm not sure if/how much oil the motorhome is using yet.

    One thing though, I suspect the 69,xxx miles are original on the motorhome, unlike the truck where it's odometer has rolled over at least once, probably twice. On the one hand yes it has sat a lot, but on the other, less IP and engine wear.

    The cabover on this sticks way out in a V shape at the tip, presumably for better aerodynamics.

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  9. jayro88

    jayro88 Full Access Member

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    That air dam doesn't look very big. The idea of an air dam is that it pushes the air to the smooth outside rather than having it hit the un-smooth underside. Here is what I fabbed up as a test. I can say that at the very least it had no ill effect on mpg. I never got to really test it on a long hwy run. I am currently in the process of wrapping it in sheet metal and adding a lip to get it even closer to the ground. Should you spend the money to build one? I don't know. Mine is built with all left over stuff, so no extra $$. If you want an improvement and are willing to spend the money I would say to add an aux O/D and then swap to an e4od.
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    [​IMG]
     
  10. Cubey

    Cubey Full Access Member

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    Yeah its not. This is the same model, just a year older. But thats about how it should look. I'd rather not have one that close to the ground anyway, I do go.a little off road sometimes where its a worn path with the hump in the middle.

    Something is better than nothing though. It blocks off the steering linkages from wind at least.

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  11. jayro88

    jayro88 Full Access Member

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    Everything is a trade off. So is an aux o/d an option? A trans swap? A low stall diesel converter?


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  12. Cubey

    Cubey Full Access Member

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    Depends on the cost. Unless the trans in it goes bad, I'm in no hurry to throw several thousand more at this poor, rotten old thing. I would consider an E4OD at that point since I'm already so in deep to this thing. Just not right now when it's still working.

    It wouldn't cost much more probably to get a used e4od, have it rebuilt and swapped in, than straight up rebuilding this C6. The hard part might be fitting one from a truck and/or from a gasser.
     
  13. Cubey

    Cubey Full Access Member

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    I got it reattached today, but a fraction of an inch off center. Oh well, its good enough.

    It's more of an air scoop for the radiator/engine by the looks of it from the side.

    20190323_164835.jpg

    20190323_164842.jpg
     
    Last edited: Mar 23, 2019
  14. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    You'd also need a computer to control it.
     
  15. jayro88

    jayro88 Full Access Member

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    Yes. A a stand alone controller for an E4OD is around $500.


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