How to better mpg on Detroit 60Series?

Discussion in 'Trucks [ Eighteen Wheelers ]' started by Trouble65, Jan 11, 2009.

  1. Trouble65

    Trouble65 Medium Load Member

    Jan 11, 2009
    Ontario Canada
    I am a small owner operator with two trucks.
    My personal truck is a 1998 Century Class. The truck is equipped with 500hp Detroit 60 series with 589,000 miles, 13 spd, 3.50 gears and 2.45 tires. Cruising at 65Mph with 80,000 pounds the truck will average between 6.5 to 7.3 MPG depending on road and weather conditions.
    The other truck is a 1998 FLD 120. This truck is equipped with a 435Hp Detroit 60 series with a complete engine rebuild with less than 80,000 miles, the 13 spd transmission, power divider, drive shaft, rear 3.90 gears, have all been rebuilt in the last 60,000 miles. Wheel alignment was performed in Sept 08 just before we installed 10 brand new 24.5 virgin Bridgestone tires. We have been pulled over by the USA DOT and the Canadian MTO and passed both times with no defects. The truck is in great shape and very well maintained. Oil changes are done every 12,000 miles. Driving at 65Mph engine is revving at 1,600Rpm. At this speed we were averaging around 4Mpg. We asked the driver not to exceed over 60Mph I believe the engine RPM to be around 1,500 with an average of 4.3MPG doing the same run as my truck.

    My question is this. Why such poor fuel mileage? What can I do to better the mileage?

    We thought it might be the driver, therefore we switched trucks for a few runs. With my truck he was averaging over 6.3 MPG. Driving the FLD I could do no better then 4.3MPG.

    If I increase the HP on the motor will that better the mileage or should I go to taller gears?
  2. Native Dancer

    Native Dancer Heavy Load Member

    Dec 28, 2007
    Portland, Or.
    Depending on terrain increasing the HP may help. Running on level ground grossed out at 60mph requires 180 hp. Re-ratioing the rears would definitely help.

    How are you calculating mpg? Filling the tanks to the top, running 1000 miles or more and refilling to top several times will give you the best results.
  3. lostNfound

    lostNfound Road Train Member

    Jun 28, 2007
    Home of the Stampede
    The rear ratios will make the biggest difference ... although a little more power never hurt either. :biggrin_255:

    Go to Roadranger's Road Speed Calculator webpage to see the road speed with the different ratios. Here's their homepage too for everything else you wanted to know about transmissions but were afraid to ask.

    BTW, I wish I could get the kind of mileage numbers you are recording with your truck. Mine is similarly equipped except that it is an '03 Classic XL on 22.5 rubber. I am trying out a Bully Dog Power Pup to see if it helps at all.
  4. Rock hauler

    Rock hauler Light Load Member

    Nov 26, 2008
    Deridder La.
    We had almost the same setup in 9200 IH, 430, 10 sp, 3.90, with 24.5. mostly got 4.8. plug in to ECM at dealer it always said 6.7, but i never got close to that checking it at the pump.
  5. Trouble65

    Trouble65 Medium Load Member

    Jan 11, 2009
    Ontario Canada
    The way we calculate the fuel mileage is basically fill up the fuelt tanks to the top. After the run we fill them up again to the top and then we calculate, Fuel used / mileage. I believe this to be most accurate.
  6. pullingtrucker

    pullingtrucker Road Train Member

    Dec 21, 2008
    Fostoria, Ohio
    That is the most accurate. What kind of Bridgestones did ya put on. Many drives have real high rolling resistance and can lose close to 1 mpg. I would also have the overhead ran just to be safe. Who knows maybe you'll find something there. Also try synthetic oils. I use Schaeffers and have had excellent results (gained about .2 mpg along with peace of mind that the oil will withstand my motor).
    ironpony Thanks this.
  7. dangerous dave

    dangerous dave Light Load Member

    Jul 20, 2008
    Houston, TX
    I just got my 60 S out of shop for same complaint. Intakes were all set too loose. .26 instead of.08. We also found some exausts off a bit, and some injector screws too. Then we preasured up charge air cooler to check for leaks. Acceptable leakage is 30 down to 25 lbs in 10 minutes I think. Sure thing someone here will correct me if I'm off. Set tire pressure at max they are labeled for. Does the unit have same airo-dynamic fairings? If you want to go 65 mph check web mentioned site above for gearing. You should be turning 1350 to 1400 RPM when at the speed you want to run. 3.90's are an off road gear. My unit has 3.70's, 10 spd OD I don't get good MPG above 62 MPH but I had problems mentioned above. was getting 6.25 best running 62, 5.3 running 65.

  8. 1999blue

    1999blue Light Load Member

    Dec 27, 2008
    I have a 1999 FLD 120, I would like to change the Air Filter to something like a K&N, I'M sure there is something out there besides paper filter? I just got this truck and I;m working all the bugs out that I can find, reading these forums sure does help, I've owned several 318's, but this series 60 is a new ball game for me.
  9. dangerous dave

    dangerous dave Light Load Member

    Jul 20, 2008
    Houston, TX
    Never ran across one (K&N type filter), post if you find. My first move after a truck trade is changing out stock muffler to Dynaflex straight thru to reduce back-preassure. Then if there's less fuel milage than I think it should get I do steps listed above in prior post. 12.7 is a little motor and small things will make a large difference. I would like to find a shop in Houston, TX area that knows best way to set all electronic controls for best economy and power if any of you know one. I generally don't go much more than 7 or 800 miles from home.

  10. 13letter

    13letter Light Load Member

    Oct 19, 2008
    Fulton, IL
    Search Cleer-Air filters. Looks and sounds like a K&N style for heavy duty apps. Havent priced any yet.
    dangerous dave Thanks this.
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