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Temperature will have a fairly large effect on fuel economy, did the average temperatures drop during that tank? Or different type of driving (highway vs. City)?
Take some time and do some reading on driving technique in the ecodriving/ hypermiling section, the average driver can gain huge improvements simply by using some simple driving techniques like trying to coast more, whenever possible. Also driving without brakes, which is also coasting and trying to not accelerate more than you have to.
There are a few (very successful) guys here with trucks, aerohead, diesel dave, big dave, slowmover, bamzippow, econoram, and I know I'm forgetting a bunch too. If you see some of their threads, they have some good info for trucks.
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