Less than 9MPG - what am I missing?!

[Number21]

I posted about this a while ago but apparently my gas mileage is only getting worse. My gas gauge doesn't work, so I've been keeping track of my mileage and budgeting 10MPG. I just ran out of gas. I'm guessing less than 9mpg right now. (on premium no less!)

My truck is pretty much stock besides the 33" tires. K20/350/TH350 with 4.11 gears. I know it's not built for economy but I really get pissed when I see somebody with a big block crew cab claiming something like 14mpg.

The engine is an old 350 (not stock) with power pack heads, that's why I have to use premium. I have an Eddelbrock Performer intake and carb, and headers, but other than that it seems pretty stock. It seems to run pretty good. I want to change the spark plugs but I took one out and it looks ok. The cap/rotor/wires are new and it has an HEI distributor.

What am I missing? Why am I getting worse gas mileage than a school bus?! I drive like an old lady.