No, that's pretty bad no matter how you slice it.
that's 454 territory at 350 power levels.
If that was a constant number, I would get rid of that pig as soon as I could.
Consider this:
I average 15-16 mpg city. That's right around double the MPG you're quoting.
I spend about $115-130 on a normal fill and that lasts me about 1 month.
so per year is 115x12= $1380
If I were getting half my mpg (your 8 mpg) my annual costs for fuel would be $2760.
Double my costs and I already pay too much annually for fuel as far as I'm concerned.
Yep, that 8 mpg truck would be sooooo gone. I need a truck but 8 mpg is too low, especially from a 350. I've got better things to spend 1300 bucks on than fuel....