Volvo VNL 670 only 5.2 mpg

Yes, I am sure. Did the max-speed calculations manually as well based on tire size 11R22.5 and Transmission ratio of 0.73:
Engine RPM x 60 / (Axle Ratio x Tire Revolutions per Mile x Transmission Ratio) = Top Road Speed (MPH) which amounts to 66.3MPH @1500RPM, pretty much the 65 I get. Also spoke to Cummins and they say that sweet-spot on engine is between 1200 and 1500 RPM but that best not to go below 1370RPM at a desired cruising speed. Cummins also claims that engine should be getting over 6.5mpg on light use, 5.5 to 6.5 mpg on normal and less than 5.5 on heavy use, whatever that means I figure running I-80 is not heavy use. Am testing now driving only 62MPH, which brings me to about 1400 rpm. If mpg do not increase @1400RPM vs. 1500 or even 1600, then there's a real problem I believe.

 

at 1850rpm you should be at around 82 not over 85, but I guess it is what it is. On my end things are looking better as we ran the rig with a light load @ 1400rpms or 62mph. Was getting just a about 6mpg, that included driving on the cross-bronx to the GW at 0 to 10 mph for 2 hours. So I'd imagine were cruising on highway at 6.5mpg or more. Seems either run at 62mph highway and lose time or change the tandems if I want to go 65 to 70.