Is this MPG normal? How can I increase mpg?

No 3.9 isn't even close to normal, but we need more information, is it C-13 or C-15? 435/1650? 475/1850? 625/2050? What transmission do you have? What rear end ratio does it have? What size tires are you running? 7700 Gal I'm guessing is some kind of fuel, if it's fuel you are hauling, are you loaded both ways or are you empty half of the time? Do you run in hilly or mountainous terrain or flat land? Do you idle a lot? I know heavy haul guys that do better than 3.9.

 

First thing I would do is check your speedometer. 3.9 is so freaking low it makes me think your Speedo is off.what I mean by this is if your Speedo is off you can be running a lot more miles than you think you are, therefore screwing up your calculations. It may need to be calibrated. If the Speedo is right, start by pressure testing the cac.

 

Your speedometer is what works your odometer. If the Speedo is off, so is the odometer. Wait a second. Do you have a gps? You can check your Speedo and odometer against its calculations.

 

I run a c13 as well. Mine gets pretty crappy MPGs too. But I think I may have some underlying issues causing it. If yours is geared like mine, 70 is waaaayyyy too fast for it. Mine does best at 55. Not too awful at 62. But anything over and I'm below 5 mpg in a hurry. If I'm heavy, I get upper 4s even at 55. I pull in Illinois (flat as can be). We run the old PeopleNet and I can bring up what my ECM says my "real time" fuel mileage is. On the slightest up grade in tenth gear, it'll drop into the mid to low 3s. Flat ground 7s and 8s. And don't get me started on if there is a wind blowing!

But like I said, I'm pretty sure mine has something going on. Just doesn't feel anything like 430 horse. Thinking my turbos may be done or there is blockage. No boost gauge. But I think that will be my next investment. I just got the truck a month ago and plan an overhead soon. No telling when it was done last.

Just putting this out there so you know that you're not alone. Lots of things can cause loss of MPGs. And I'll keep throwing money at mine till I get them all back. LOL