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Thread: Max fuel economy for the 6.5TD

  1. #41
    moondoggie Guest

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    Good Day!

    My memory is pretty foggy, but I thought aero drag was proportional to the square of speed, not the cube, but if wrong, it won't be the 1st time...

    From REALLY foggy memory, they used to say that the best mpg generally occurred where rolling resistance equalled aero drag, which was (for most cars) ~ 35 mph or so. At this point, aero drag was increasing so quickly it dominated mpg.

    In fact, I also remember reading that rolling resistance was generally constant, & actually drops some at higher speeds. It drops so little that it's massively overwhelmed by aero drag increases, however.

    I have no idea where drivetrain friction works into this.

    My personal experience is that my mph drops dramatically as speed increases, which would support aero drag being the dominating factor.

    Keep in mind, ALL the above is from the memory of a guy that can't remember what he had for lunch yesterday.

    Keep the good ideas coming - mpg is my life.

    Blessings!

    Brian Johnson, # 5044

  2. #42
    rjschoolcraft Guest

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    The drag force does go up with the square of the speed. The power required to propel a vehicle against that force goes up with the cube of the speed, as was astutely pointed out above.

    Back when I worked on the Joint Strike Fighter preliminary design, I had to do a bunch of preliminary sizing on fans and clutches as we were trying to settle on a configuration. For the clutch and gear train, I needed to know how much power it took to drive a fan at various speeds. The aero guys gave me curves of windage torque vs. speed and windage power vs. speed... Torque increased with the square of the speed and power increased with the cube of the speed. This directly correlates to what is being discussed here.

  3. #43
    Join Date
    Mar 2000
    Posts
    19

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    From what has been discussed here, it sounds like an electronic air suspension that will lower your truck a couple inches for highway cruising might not be a bad idea. You could also have normal mode for stock height and a 4x4 mode that raises it a few inches for off-roading. IFS throws a wrench into it, but you could always do a full axle swap.
    1999 K2500 Suburban LT w/3.73 gears
    265/75/16 Bridgestone Dueler A/T Revos
    SS oil lines & HB7 cooler
    More to come...

  4. #44
    moondoggie Guest

    Post

    Good Day!

    MJEasly said, "...it sounds like an electronic air suspension that will lower your truck a couple inches for highway cruising might not be a bad idea." I think there may be cars on the road that already do this. In fact, I remember reading somewhere a long time ago that belly pans would help mpg too, they're just impractical.

    ronniejoe: Next time we meet, I'm gonna get you to explain to me (if that's possible, I'm pretty dense ) how aero drag increases as the square, but the power to overcome it increases as the cube. (I suspect it'll either be a forhead-slapper or I'll never get it.)

    Blessings!

    Brian Johnson, # 5044

  5. #45
    chickenhunterbob Guest

    Post

    My driving is pretty much strictly scenario #2.

    City miles are minimal, mostly highway, and no towing to speak of.

    On the highway I can average 23 - 25 MPG in Canada, with occasional highs of 27. This with the tach at around 1900.

    On the US interstate I-29 two weeks ago between here and Nebraska, I recorded a dismal 19 (75 MPH speed limit and a real strong south west wind). Tach on 2400 at 75 MPH, the old truck never knew what hit it. And me as a fuel mizer, I was ashamed of myself, at least somewhat, but shaved a couple hours off the time to get there.

    Not so big of a rush to get home, even though took an hour or two longer, slow to 65 MPH and 24 MPG, tach still around 2000.

    A previous post a few months back (may have been Moondoggie, not 100% sure) suggested before cleaning out the bank account to overhaul your truck to increase MPG, figure how much you could gain vs how much you spend.

    I did the math at the time, and for me with the price of fuel as it was a couple months ago or more when that thread was up and running, if I spent 1000.00 on a 1 MPG increase, I could recover that amount in fuel savings in 140,000 miles. Well, not likely the truck will live that long, so for me, any opening of the engine, aftermarket exhaust, etc. would never pay for itself in fuel.

    lots of mods "claim" higher mileage, but if you read this board, everyone who does any mods at all seems to gain weight on the right leg immediately, and I haven't heard of any drastic fuel mileage increases.

    Maybe if my truck was newer, and had 300K ahead of it, there may be some benefit. But to me, I CAN drive 100KPH, and will, and will hopefully continue to be happy with the mileage.

  6. #46
    moondoggie Guest

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    Good Day!

    chickenhunterbob: Is that mpg in US or Canadian gallons? If it's US, that's some of the best mph I've ever heard of, & I'm WAY jealous.

    Blessings!

    Brian Johnson, # 5044

  7. #47
    rjwest Guest

    Post

    With my truck camper, The fuel rate increases Dramaticaly from 60-70 MPH,

    If I tuck in behind a Tractor/Trailer, I see a
    2 MPG increase ( Instant readings )
    13.0 to about 15.0
    Also ( my opinion ) That if I can get a run at a hill and ease off the throttle on the climb,
    I can get a slight increase in mpg

    This technique worked very well on my old 6.2 N/A
    with the mech IP, but quite difficult on the elect
    IP.

    My Total average with a 3000 lb truck camper is
    slightly less than 13.o mpg US

    The headwinds KILL ME though....

  8. #48
    chickenhunterbob Guest

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    Moondoggie,

    Those calculations are in imperial gallons.

    My mileage, X .833 would give the corresponding figure in US gallons. Miles are the same wherever you go.

    Up here, fuel mileage is calculated in litres per 100 kilometers, but I'm too old for that, I always convert to MPG which is a meaningful figure to me.

    Bob

  9. #49
    eracers999 Guest

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    Dont have hard no's yet but after i built this new engine and have over 1k mi on it i have started to check fuel mi. The old motor was getting fuel at 325 mi. The new motor is at 302 mi and the fuel guage is just under a 1/2 tank.
    That is a lot more fuel and i have a lot of messing around on this tank and was expecting the no"s not to be too good. So far i am very suprised at how much less fuel has been used.

    Engine has much better torq, and cruises at 70 mph with muck less effort. So tickled am i over the end result of this project.

    Kent

  10. #50
    moondoggie Guest

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    Good Day!

    [i]

  11. #51
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    Bumped up for Frijoli.

  12. #52
    rjwest Guest

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    16 ga you might want to rethink the reason for the more time/fuel used in flight , not related to drag, just amount of time in Head wind condition.

  13. #53
    Marty Lau Guest

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    Originally posted by rjwest:
    16 ga you might want to rethink the reason for the more time/fuel used in flight , not related to drag, just amount of time in Head wind condition.
    Yes I know I was a CFII for a number of years, but when you figure fuel consumption it's with regard to time in Aviation. The point is that many people are on a false belief that if you have wind the round trip is the same just on way is faster and the other is slower. They do not equal out. I have had people insist that if they go faster that their fuel consumtion for the trip will not increase beause yes they are burning more fuel per hour but they are covering more ground in that hour.
    I have also had people tell me that wind does not make a difference in the time enroute on a round trip because the faster leg negates the slower leg..... both false. I have had have to go as far as make people (or do them for them)do the calculations to prove the point.

    [ 05-13-2005, 08:32 AM: Message edited by: 16ga SxS ]

  14. #54
    rjwest Guest

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    Ditto: Agree, same with 90 degree crosswind
    still more fuel used...Come to think of it.I guess it works that way in a truck also.

  15. #55
    GMC Hauler Guest

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    I recently changed out the rims on my Suburban to new rims that were take off's from a newer truck (click on my page to see pictures). The rims are 20 # lighter each. I changed tire sizes at the same time. I had 265/75R16 load range C. I wanted to put the correct load range on, and a slightly narrower tire, so I went with a 235/85R16 load range E.

    Tire size is barely different, but I wanted to calibrate the VSSB (vehicle speed sensor buffer) anyway. Last time I calibrated it, I used road mile marker signs. This time, I used a GPS unit.

    I found that my speedometer was reading 1 1/2 to 2 mph slower than the GPS reading. I readjusted the VSSB (per 1999 feature articles and product reviews) several times until calibrating the speedometer dead on. The spedometer reading slower than actual speed has several affects which affect gas mileage.

    The first is that indicated miles will be lower than actual miles. This will cause your computed mileage to read lower than it actually is. Readjusting the VSSB will cause this to read correctly. This doesn't improve actual mileage.

    Secondly, most of us drive to the speed limit, or a value above, e.g 5 mph above posted speed limit. If your indicated speed is low, you will in actuality be driving faster to make your indicated speed meet your desired drive speed. This has the effect of making your actual speed higher which reduces your gas mileage.

    I was lucky to have a VSSB that was reprogrammable. Maybe those of who have changed tire size or arent sure should take a look at our calibration.

    [ 06-06-2005, 10:10 AM: Message edited by: GMC Hauler ]

  16. #56
    moondoggie Guest

    Post

    Good Day!

    More Power: Any chance this will become an article? (You know me & mpg...)

    Blessings!

  17. #57
    Join Date
    May 2001
    Location
    Edmonton, Alberta, Canada
    Posts
    184

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    I put on a lot of km's so fuel economy is also important to me. I would also like to see more information on this subject. I believe one was in the works regarding fuel economy and different fuel additives. So far I personally have found Power Service to give me the best, 26.2 mpg imperial....21.8 U.S. mpg at 65 MPH empty.

  18. #58
    Marty Lau Guest

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    Originally posted by kowsoc:
    I put on a lot of km's so fuel economy is also important to me. I would also like to see more information on this subject. I believe one was in the works regarding fuel economy and different fuel additives. So far I personally have found Power Service to give me the best, 26.2 mpg imperial....21.8 U.S. mpg at 65 MPH empty.
    Kowsoc; your doing real well give yourself a pat on the back.
    How about putting your truck info in your signture so we can see what your driving Sir?

  19. #59
    Join Date
    Apr 2004
    Location
    Virginia
    Posts
    127

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    All these numbers everyone is posting is making me sad. Our first road trip in this vehicle a month ago netted the same fuel econ we get around town. 15.5 mpg. Now we were running 70 t 75 all the time, but no trailers and only 2 adults and a 2 year old so not much weight.

    I have never had my injection timing checked. I think that might help. I didn't notice anyone talking about that in this thread, but I might have missed it.
    Eric
    '94 4x4 2500 6.5TD Suburban
    http://sofadog.net/6.5TD/ (work in progress like the truck)

  20. #60
    Join Date
    May 2004
    Location
    Southwestern Michigan
    Posts
    138

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    Hey ess - I have a 94' burb and I only average 15 to 15.5 - with 3.73's - I have done the cooling mods so far but next I want to get the truck breathing, (intake and new exhaust) new injectors and glows (I'm pretty sure the injectors are old and worn) than gauges and a new chip. I think that once I get some new parts on mine, the mpg will go up.

    KEvin
    2007 classic sierra 6.6 duramax - 4x4 crew cab burgundy red1994 k2500 6.5 TD suburban with remote PSD 97 cooling mods JK fan and clutch SOL/D (at least for summer) hi output alternator DSG gear drive DSG oil cooler 4" exhaust 1983 International 3 cylinder diesel tractor (izuzu Diesel) 1990 1500 gas 5.7 suburban 1995 John Deere 310D Turbo Diesel Loader/Backhoe house addition/move

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