Needing help from 5.6mpg to 6mpg+++

Anything under 1300 rpm is lugging the engine. And u would be doing major damage to the engine overtime and run into head and liner problems . Cruising is one thing at that rpm and if u bump rpms to 15 or higher when getting into a hill. But if u pulling up a hill at 13 or less "because thats the torque range" u just asking for problems. U understand ur engine red lines at 2200 or so. If u running 1300 or less u are leaving all that power between 15 and 2200 to waste and thats where most engines like to run. Ran so many pieces of equipment and now trucks and when u needed it to work u would get into the rpms not the torque. Idk a good mechanic that would tell u to run anything less . Just my opinion from my experience . U can drive how u want

 

Lugging boils down to your engine type. My Detroit engine manual says to generally downshift between 1000 and 1100 rpm on the DD15 and DD16. However, if you have a DD13, then the recommendation is downshifting between 1150 and 1250. I posted images of my Detroit manual here (comment #15):

Uphill Shifting

 

On a mechanical engine I'm with you.
But on a modern electronic engine, No

To sum up, torque plays a greater role in towing than horsepower. This is because of the 'low-end rpm' generated by the higher levels of torque, which allows the engine to easily carry heavy loads. A high torque vehicle will be able to tow trailers or other objects with an extremely low value of rpm.May 20, 2016

Without doing a bunch of math on a 1000 mile trip how many more rpm at 2000 rpm vs 1300 rpm

630.000 revolutions in 1 trip.

 

And lugging to me means overfueling a mechanical motor and needs rpm to blow the excess fuel out the stack and an electronic engine the computer controls that., so not much of an issue.

 

Well both modern engine and old school engines are mechanical. Pistons still move up n down . The only differenece is every system is montiored and calibrated with electronics now . But Like i said. Not gonna argue. U run how u want. But id be ###### if i ever abused an engine like that. We used to run some of our equipment up in the 3000 to 3500 range running ptos and hydraulics. And i got close to 20k hours on equipment not including what i put on trucks but it aint that much only been out here a couple years. But i paying the bills on my equipment so i gonna run it how i want

 

my guess that as a fleet truck it the overhead may have never been touched. It wouldn’t hurt to spend a few hundred to make sure it’s set right. Some air filters can be hard to check, especially the ones that flow from the inside out, the outside always looks clean.

 

Couple things, overhead is a good idea and will probably cost less than a fuel up. Do me a favor and look at the driver side door sill. What size tires did it come with? These old fleet trucks used to run super singles but they throw in a spacer and slap duals on them to sell to the used crowd. What size tires are you running?

 

You may be squeezing almost all you can get out of that truck. Your profile says MO. If you’re pulling heavy wind catching loads through hills, you’re not doing bad. You can eliminate winter idling by installing a Webasto bunk heater. They work great, don’t cost a lot, and require minimal maintenance. Watch your boost gauge, fuel generates boost, so the more boost you see the more fuel you’re using.

 

My truck is completely un governed when I go up a Hill I keep the RPM a little higher on flat ground I-shift at 13-14

 

You never answered the question of what psi you run your tires at. A lot of people, including mechanics don't know how to properly read a sidewall. Im not gonna go into how to read it, but take a look and see if you see a maximum # listed for pressure.

Anyways I don't care what anyone says, I run my tires (and I run FB as well) at 120 psi. I also run good tires, Michelin steere and currently bfg drives which are what the co puts on the truck. In the last 13mo I havent had one tire fail from heat or low pressure. I actually haven't had a tire blow at all, but I dont want to jinx myself too much.

Low pressure and heat will kill a tire fast. Lots of rolling resistance builds heat and poof. Bye tire.

Another thing is Id bet your engine is set with a low hp/tq rating, which will cause increased fuel consumption because you're always in the throttle more.

I run a 386 Pete with an ISX at 450hp. It pulls hills great and Im not standing on the throttle constantly. Since the inframe 16k miles ago, I am averaging 6.6mpg with lots of tanks up around 7. If I can get it rerated soon to 515, I expect my mileage to go up about a quarter mpg, maybe a bit more.

You have to learn how to drive your truck. Spinning the engine way up at 60mph in 9th won't get you good mileage.

I also dont lug my stuff down to 1000-1100rpm if I can help it. 1250-1700rpm every day.

Most everything else posted I agree with as well.