Disappointing MPG.

[rscam]

I have a 69 GMC stepper. It has a 355 with roughly 300hp with an Edelbrock 600 carb. I have an NV3500 with 3.08 rear gears and 28 inch tires. 60 mph is a cool 1600 rpms and it doesn't struggle to maintain that speed. I just got my speedo working so I checked my mileage and got a disappointing 12.25 MPG. I drove the truck very easy and didn't get the truck over 2200 rpm when shifting. I was hoping for 14-15. Can I tune the Edelbrock for better mileage? I just put it on straight from the box and really only adjusted the air mixture and idle.