ISX fuel mileage help

[jmickulin]

New to the forum and looking for some suggestions. In March I bought a 2007 Peterbilt 387 with an ISX Cummins in it. I pull a reefer from Idaho to Arizona, then to Colorado and back to Idaho. The truck gets terrible fuel mileage and I am almost at a loss with what else to do with it. When I bought it and ran it on the first trip it got 4.5 mpg. I took it in and had an overhead done which brought it up to 5.1 mpg for the next trip(better but not great at all). After a couple trips and a little research I unplugged the EGR just to see if i saw any improvements. Besides my reefer not being covered in soot, the first leg of the trip i got almost 6 mpg. Unfortunately, on the second leg of the trip I was down to 5.5 mpg, and right back to 5.2 mpg by the time I made it home.

I found this forum and started doing more research. I saw a post that talked about the IMAP sensor and cleaning the intake tube out. I pulled the sensor and it was completely plugged off. I cleaned the intake and went and bought a new genuine cummins IMAP sensor and replaced the intake temp sensor while I was there. Well, first leg of the trip I managed 7.9 mpg and I was JACKED!! But, like each time before, the second leg of my trip was down to almost 6 mpg and by the time I fueled again at home it was 5.4 mpg. I have been on 3 more runs since and I am sitting right at 5.1-5.2 per trip. This last run I plugged in the EGR again just to see what would happen with a functioning EGR and new sensors and it was right back down to 4.7 mpg.

I don't understand why every time I do something I get a temporary gain in mileage and then it just drops right back down no matter what I do. I talk to guys pulling reefers with ISX's and none of them get near as bad as mileage as I do( I know its hard to compare 2 different trucks). The truck has a 13 speed and 295/75 22.5's and I need to check my rear end ratio. I run 100 psi in all tires except the steers in which I run 110 psi. I keep the rpms under 1450 and try to drive by the boost and pyro gauge. What's your thoughts/suggestions on this? What are my next steps?