Volvo VNL 670 only 5.2 mpg

trailer, terrain, how fast, winter or summer, average length of load

 

That's a lot of truck. 249" WB for a 670? 525HP & 3.73 ratio, 5.2 is probably pretty good. Mine was 3.42 ratio and 430 HP 10speedOD, 6.5 MPG driving in the low to mid 60's.
That 3.73 ratio would scare me.
Mine was 670 but 220' WB. Yours 2-1/2 feet longer for some reason. Is it an old heavy hauler?

 

Ah, yes, that probably does have a lot to do with it. I know a lot of engines if you don't use new factory injectors, they just never work right. I've heard so many guys with problems because the shop puts in remanufactured injectors. And I also have heard so many say the Cummins EGR is a nightmare as well. Mine was Volvo D13.

 

Yes I have the same truck getting 6.8 to 7.1 driving 62 mph and had the same thing happen went down to 4.8 after many trips to cummins and no fix I changed all the sensors on the intake and exhaust sorry I can't tell you which one was bad but my mileage come back to normal again. Something that cummins never did they just took my money. Oh my mine was 565 hp but all other spec the same as your truck. Dry van average load 30k to 42k

 

Wow, I can break 5.0 mpg this time of year pulling 140,000 lbs super-b train with my not at all aero dynamic Western Star.

 

Replace boost, delta egr, exhaust pressure sensors. These sensors plugs due to carbon and cause bad fuel mileage. Clean egr valve. It can cause bad mileage too if your egr valve sticking. If its 2011 clean or replace doser injector. Check with insite dpf differential pressure should be arround 1inHg, run forced regen and check numbers.