CM870 fuel mileage

[Vjex]

Hei guys,I have 07 volvo cm870 950000 miles.I thought I was getting good fuel mileage (7.5-8.5) according to my dash computer.I would reset it at every fill up and would break 8 mph easily. When filed for ifta,my avg mpg came out to be only 6.7 for whole quarter.Its not too bad,but I do a lot of light loads,have tripac apu,so I dont idle at all.When started comparing dashboard computer numbers to actual pumped gallons,I realized that it is off by roughly 20%!Lets say since last full fill up comp shows I burned 178 gall,at the pump to fill'er full I'm ending up with 212 gallons.I didnt expect that thing to be super accurate,but to be off by a 1/5 -thats a lot. Anyways,does it mean I need all actuators replaced?