Help me with my C15 for better mileage ??

I've found with mine that I consistently hover around 5.7-6.0 overall fuel mileage. A lot of it has to do with how often I'm speeding up or slowing down for passing or traffic.
For example: Filled up in Austinburg OH, drove loaded to delivery west of cleveland and then headed back east empty stopping at Oneonta NY. Around 700 miles and I only used 94 gallons which came out to 7.4 mpg. The next day driving home (still empty) I was in morning rush in MA, lots of stop n go, and just heavier traffic than the day before. Used another 50 gallons to drive 366 miles.

I have an '07 379 with a fresh built MXS 550/1850 and added bullydog power. New cac, iva's, fuel pressure reg, etc.