Paccar MX13 Fuel Mileage, how can I get to 7mpg?

They lied

 

No they didn’t.

 

I have a 2014 Peterbilt 386 with an MX-13 EPA 10 and 10 speed transmission. I get various 7.2 to 7.8 MPG empty and 6.4 to 6.7 MPG loaded. I'm running 22R 22.5 tires and keep them rounded 95 psi. Recently Rush Peterbilt reprogrammed my ECM with PACCAR headquarters, that changed or improved my MPG a few tenths per mile, so I would suggest that you try that to improve your MPG and overall performance. It made a difference on my truck.

 

I drove a 2016 Peterbilt 386 day cab for a local company a while back and it sucked on fuel too. Most of the loads were super light, fuel tanks, battery and tool boxes from fabricator shop to Peterbilt plant. Only had one tank and it drank the hell out of that fuel. It was an MX-13 engine with around 680,000 miles if I remember correctly. I don't know what rear end the truck had though, I am sure it wasn't 3.73, it didn't pull like it had 3.73 when I would haul heavier loads with it.