Question regarding MPG and Rpms VS Turbo Boost

Discussion in 'Experienced Truckers' Advice' started by jcbarberio, Dec 8, 2013.

  1. jcbarberio

    jcbarberio Bobtail Member

    Dec 8, 2013
    Hello fellow drivers, I have been driving for over 9 years and still consider myself a rookie driver. I have always been a company driver and have plans on purchasing a truck in the future. I have always driven a standard company truck, Mack, International, Freightshakers etc. I currently took a new job that has me running a 2012 Peterbilt 386 with a Cummins ISX 15 engine. The truck its self is all decked out with the O/O Specs and I have some questions regarding mpg's and running efficient as possible.

    First question is that I have always been taught to run slower 59-62 in a 65 zone for better fuel economy, (I have done this in the past and achieved a 1 mpg or greater in my past trucks), well this truck i'm running I have a hard time achieving 6mpg running empty (Tanker Work). It seems that if I run average of 62 I only get 5.8mpg and if I run 65-68 I hit 6 to 6.2. When I'm heavy 75,000 lbs it's a disaster, maybe 4 at best. I have always been told to run with the turbo boost at bear minimum as higher the boost psi the lower the fuel economy. Maybe some fellow Peterbilt drivers can tell me what is better, running lower rpms with a higher Turbo Boost psi (20 psi) or a higher rpms with a lower Boost PSI (10psi). Like I have said I have been taught lower RPMS but never had the luxury of having a Turbo Boost Gauge. I would just like to run as efficient as possible for my company and learn so by doing it for when I purchase my truck.

    Second question is when I'm running heavy/pulling a load, I get a nasty rumble in the drive line just under my feet when pulling a hill, I have checked the drive line, Joints and they are all tight, Is this normal for this truck, (If so that sucks) any ideas what to look for. The truck only has 82,000 miles on it and unfortunately has been abused by former drivers. I was tossed the keys and said just take care of it with no previous history of it.

    I have a nasty vibration in the dash also when I get the rumble sounds of the drive line, I just would hate to say "This truck sucks" and keep a positive attitude.

    Any help / suggestions would be appreciated. Thanks in advance
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  3. CDL1968

    CDL1968 Medium Load Member

    Sep 27, 2012
    If you ever get 6 mpg of less on any truck you sould have the truck checked. It may have an issue with the computer or a sensor. The exception would be if your truck is more that 15 years old or you doing heavy haul like over 150K or 175K pounds gross.

    Most companies and my company will fire a driver for having 6 mpg on a regular bases. With the on board computer I can tell if a driver is waisting fuel with excessive idle time or the the driver just doesnt know how to shift or if these is an issue with the engine. If the truck needs repair I get it repaired. The driver is the issue I first give a Letter of Notice putting the driver on notice and explain what the issue is. The second time I give a Letter of Correction again puuting the driver on notice with an expliantion of what they are doing wrong and a deadline for the driver to make the correction. Thrid time I release the driver from employment.

    All my trucks are less than 5 years old and normally haul 79K - 80K gross all the time because we are bulk and get paid by the ton or gallon. All my truck GenPaks (Mini-diesel) generators to keep drivers comfortable during none driving hour. As a fleet we strive for 6.8 mpg and normally hover between 6.8 and 6.9 mpg. One (1) mpg over a year or five years can mean a big differance. Anything under 6 mpg should be throwing up red flags at dispatch and anything under 5.8 - 5.7 should be blinking red on the screen in the companys Shop Dept.

    Turbo pressure is a nice toy on your dash but its useless. It only help you see when a turbo isnt working well. Turbos unlike Ultra-Chargers are not controled by the driver. Turbos work by the exhaust from the engine spinning an air intake fan therefore increasing the fuel to air mixture in the engine which increases fuel effiency and horsepower.
  4. puncher

    puncher Medium Load Member

    Feb 12, 2010
    Since you had to increase your speed to increase your mpg's, then you probably had to increase the rpm's, and hence you've probably found the sweet spot for this set-up. My guess on the vibration would be that you may be lugging it, try dropping a gear and run a higher rpm to see if it helps. Good luck.
  5. Mountain Hummingbird

    Mountain Hummingbird Medium Load Member

    Oct 14, 2012
    could also be a problem with the rear leveling valve, that will cause a vibration.
    puncher Thanks this.
  6. Pumpkin Oval Head

    Pumpkin Oval Head Road Train Member

    Jun 24, 2010
    Scranton PA
    I had a rumble that would come and go, that you could feel in the truck. I had drive tires on the right side on one axle running about 15 degrees warmer than the left side. I used an infrared thermometer to check tire temps when loaded to see if any are running warm. I thought maybe it was a bad set of tires on that axle or the drive axle out of balance. Turns out the rear drives needed alignment and the problem was fixed. This was on a Peterbilt that was a 379 built in 05. This was about 2 year ago.
    puncher Thanks this.
  7. 25(2)+2

    25(2)+2 Trucker Forum STAFF Staff Member

    Sep 18, 2006
    the road less travelled
    Those trucks are not what their predecessors were. How often are PMs done, and where? Are there any check engine lights or codes being shown?

    Finally, what are you pulling? Report the vibrations and how it runs, you are still a company driver.

    For what it's worth, the newer engine manufacturers will tell you to run it low RPM and avoid shifting unless it is absolutely necessary. Ok to lug in other words.

    This is across the board, any make, any engine. I'm not convinced it is the right way, and I'm glad I don't have the red boat anchor under the hood of my company ride.
  8. jcbarberio

    jcbarberio Bobtail Member

    Dec 8, 2013
    Thanks for the reply, I have always tried to achieved top mileage on all the trucks I have driven. In the past I have normally exceeded 7mpg and upwards to 8mpg with no problems. We currently run either empty or loaded (drop and hook) so we can deadhead 200 miles empty then 200 miles at 80,000 back to the terminal. This truck I'm driving is a day cab so Idling is kept to 3% or less per fueling. I will not Idle any more than is necessary, I use the onboard dash display to track what the truck is telling me (mpg's, Current Economy etc). The truck just had new tires put on prior to me coming on the job and they are rather a higher Rollin Resistance tire, I think around 140, they are a Bridgstone M726el on the drives. I am used to running tires that have lower resistance like 105 to 115, I also keep them exactly even in psi all the way around. I was thinking this could be some of the mpg problem.

    As for the vibration, the truck drives smooth at any speed but the moment you "Power Up" the engine the vibration / rumble gets worse even with higher rpms. I am at the point where I will speed up a lot just to make a hill with very little use of the throttle cuz it absolutely drives me nuts.


    This is exact words from the OWNER of the company, I don't care where you buy fuel, get what ever you need for the truck from the truck stop and I think we service the trucks at 15,000, Go to Peterbilt and they do all my work. I was blown away by his business ethics and can't figure out how he has stayed in business. I can go on and on about his biz ethics, but I won't bore you to death. I have a recall scheduled this week on the DEF tank and plan on talking to the service Mgr about this truck but was looking for some thoughts prior to going in for service.
  9. jcbarberio

    jcbarberio Bobtail Member

    Dec 8, 2013
    The company has absolutely no idea of when pm's should be done, they do what the dealer tells them is needed, I'm still trying to figure things out with this small company and as for codes, engine lights, nothing is showing. We do drop and hook tanker work, so either empty or loaded 80,000+. I have read Cummins website and they rate the engine at best 5.5 to 6 mpg (light duty). We have 5 other trucks in the fleet and they to don't do much better than 6 and they run much harder than me. I have a hard time believing this is the best we can get as a fleet.
  10. GasHauler

    GasHauler Master FMCSA Interpreter

    Oct 23, 2005
  11. marineman227

    marineman227 Dock Waterer

    Jan 26, 2008
    Neenah, WI
    Like someone else mentioned I think you found the sweet spot as far as speed. I have been testing different speeds in my truck and right now my best fuel economy comes at 68 which keeps it right at 1500 rpm. Granted my best fuel economy is averaging 5.7mpg. I have a check engine light on and it's running on reduced power so keeping it spun up it gets a little more power which seems to be what's helping.
    puncher Thanks this.
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