1985 gmc swb 305/700R MPG

[68c10owner]

Since my fuel gauge isn't working I tend to just put in $10-15 every couple days but I decided to do a test to check the mpg. I filled up and drove it fairly normal. Some around town but mostly freeway with a couple heavy runs merging onto the freeway otherwise normal acceleration and cruising around 65-75mph. Truck has a E-qjet, fresh 700R with stock converter, 3.08's and 30" tires. Not ideal tire and gearing but that's how I got it. Just going fill up to fill up I'm getting 12.81 mpg. My old 83 c10 swb with the same engine,trans and gearing but with stock shorter tires was getting 14.5-15.8mpg. I've done a complete tune up but could probably use a fuel filter. Thinking my tire size/gearing is killing my mileage. I think I need to either put some gear in it or use shorter tires. Can't afford either one right now. Was thinking of goin with a stock 350 GM crate engine but worried the mileage might be worse. I would think it would be better since it would make more power and require less throttle to get moving but I don't know. Not really sure what I'm asking here, if anything. Mostly just thinking out loud. I would like to hear from other members with similar drivetrain as to what your MPG is.