Bad mileage?
 

So I have a 96 12v single cab 2wd and I'm taking a long trip to southern il so I thought it would be a good time to get a good read on mpg. There is a headwind but I'm only managing 13 miles to the gallon going about 70-75. Why is my mileage do bad? For mods I have no fuel plate, 4k gsk and 4" exhaust. Also I'm averaging 10 psi on the boost gauge, could the boost be killing my mpg? Also I'm unloaded truck weighs about 6900-7000

Also running about 2100-2250rpm

Sounds like you may have 4.10's for gears, I'm several hundred rpm lower at those speeds with the higher gearset. Still, 13mpg seems unusually bad. You could probably pick your average up substantially by bumping your pump timing up. If it's never been checked, it's probably slipped lower than it should be too.

10PSI at 2250?

that's. . .that sounds low.

as for 75mph, that's 121kmh. I'd be doing 2100rpm at that point and I run 3.55s.

Jbear: you must run larger tires?

285/75/16's. I'm sure it helps. I'm just a tick under 2K @ 75mph.

Also, unless I'm trying to maintain that speed climbing a hill, I'm usually closer to 5psi or less. I definitely don't average anywhere near 10psi on the highway.