Low fuel economy

[Keving]

When I first bought my truck it averaged around 6.5 to 7 mpg. nowadays I'm lucky to get 6 with most of my hand calculated stops being 5.5, I used to drive with the cruise set at 70 but I've been trying to slow it down and see if it helps with no real success. Anyone have ideas on things I can look at? The truck has now been demandated by mr hag however I noticed this drop in fuel economy before this was done.
I've since replaced the air filter and fuel filters which made no change and in the last few months I replaced the imap sensor which also made no change the truck had a detroit dd13 and a 10 speed auto if it helps thanks.