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Thread: FG fuel economy

  1. #31
    Join Date
    Oct 2010
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    Victoria, BC , Canada
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    11
    Well if torque is flat from 1,600 RPM to 2,400 RPM, then cruising at 2,400 RPM chews up way more diesel for no added torque gain. Fuel consumption is closely related to engine speed.

    What I am suggesting is to select tire diameter and diff ratios to allow the truck to cruise comfortably at where the engine operates most efficiently (uses the least fuel for max torque output). This will vary truck-to-truck somewhat, since it depends on an individual's trucks rolling resistance (weight) and aerodynamic drag (wind resistance) and the average grade (flatness).

  2. #32
    Join Date
    Nov 2009
    Location
    Perth, Australia
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    310
    What RPM you guys turning??
    The old FG649 trucks (like mine) turn about 2900-3k rpm @ 100km/h. the newer FG84 turns around 2600 (IIRC). I run 38.5" michelins on my truck, which bring it back to 2500, which is 22% overgeared.
    You could not possibly run any taller gearing/bigger tyres than what i have without changing; a) The gearbox, because the ratios are too far apart on a stock truck, they are only just bearable with the 38s.
    And b) engine power/torque. Fuso's are not known for thier brilliant performance, this becomes exponentially worse with taller gears/bigger tyres.

  3. #33
    Join Date
    Mar 2010
    Location
    Brisbane, Australia
    Posts
    350
    What RPM you guys turning??[/QUOTE]

    Our fg649 crew cab 2003 2700 rpm at 100km/h with 285/70R19.5 tyres on it.
    Avg 17 lts per 100km.
    Last edited by PKDreamers; 11-01-2010 at 09:50 AM.

    Pete and Kel

  4. #34
    Join Date
    Oct 2010
    Location
    Victoria, BC , Canada
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    11
    These trucks seem to be primarily built for construction/landscaping/utility uses rather than efficient highway cruising, and are geared low (axle ratios 5.7:1 ?). That is good for low speed, heavy load applications, but it also leads to excessive RPM for moderate load applications, say relatively flat highway cruising.
    It appears the truck needs an overdrive for overlanding applications to get the highway RPMS down (say to 1,800 RPM) at the lower end of the torque curve to help fuel consumption.
    Larger diameter tires are really a second best approach to achieve higher highway gearing, because they add to rolling resistance, slow acceleration and add to unsprung mass.
    The low diff ratios are great off the line and off-road, but have a large fuel consumption penalty for moderate load highway cruising, where I bet these rigs spend 90 % of their time.

  5. #35
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    Nov 2009
    Location
    Perth, Australia
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    310
    These trucks seem to be primarily built for construction/landscaping/utility uses rather than efficient highway cruising
    yes. also explains small wheels and lack of power in standard form.

    axle ratios 5.7:1 ?
    affirmative, on the FG649 trucks. new trucks have 5.25, with a bigger motor and lower redline (although max useable rpm of around 3k is roughly the same anyway)

    It appears the truck needs an overdrive for overlanding applications to get the highway RPMS down (say to 1,800 RPM) at the lower end of the torque curve to help fuel consumption.
    They come standard with an overdrive of 0.75 (or so). like i mentioned in my last post, to get any more than 20-odd % overgearing would require substantial modification, because the engines simply dont have the outright torque to pull low rpms at high speed

    Larger diameter tires are really a second best approach to achieve higher highway gearing, because they add to rolling resistance, slow acceleration and add to unsprung mass.
    Yes and no. dont forget, these are still offroad trucks, the small (7.50R16) tyres they come with are as useless as mudflaps on a speedboat at anything, going to larger dia wheels helps not only with cruising speed/rpm, but helps enormously with offroad ground clearance and traction.

  6. #36
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    Oct 2010
    Location
    Victoria, BC , Canada
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    11
    Sounds to me like engine is too small, if you have to run at the high RPM part of the torque curve, which burns a lot of fuel (2,600 RPM vs 1,800 RPM.) Have you tried bigger exhaust and increasing turbo boost?

  7. #37
    Join Date
    Oct 2009
    Location
    Southern Oregon
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    71
    I think that it's more a matter of small injectors, low boost and mild tuning. With mild mods my '95 5.9L Cummins puts about 300 HP to the wheels while pumping 34 lbs of boost, and it's pretty stone age design compared to the 4 valve Mitsu motor. I'm sure with the right ECU, injectors and turbo it would put out much more power. Not sure how the trans and clutch would like it though.

  8. #38
    just out of curiosity, what would a 5 ton ford or dodge or other make get for fuel consumption? What do some folk on this site with a camper on their truck get for mileage, I know my brother in law with his f350 , when towing his fifth wheel gets abysmal mileage, s how are oth3er units doing??

  9. #39
    Join Date
    Oct 2009
    Location
    Southern Oregon
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    71
    My Dodge Cummins, standard cab with a utility box and a NCO Alaskan weighs 10k pounds with 90 gal of fuel. Cruising at 65 to 70 mph I get about 14 mpg. I'm sure there will be times in the Fuso that I'll be wishing that it was Cummins powered.

  10. #40
    Join Date
    May 2008
    Location
    Cairns FNQ
    Posts
    379
    Quote Originally Posted by justduck View Post
    My Dodge Cummins, standard cab with a utility box and a NCO Alaskan weighs 10k pounds with 90 gal of fuel. Cruising at 65 to 70 mph I get about 14 mpg. I'm sure there will be times in the Fuso that I'll be wishing that it was Cummins powered.
    So why bother getting a Fuso, stay with Cummins or what ever.....
    I used to be indecisive, but now I'm not so sure.

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