What kind of fuel mileage are you stock, or lightly modded late model 4.6's? I have a friend looking into buying one, but the projected 17 mpg is a turnoff... Is this close or what?

. I get about 260 miles on about 13 gallons (~20-21mpg). my bf (stanglou) gets the same and he has pretty much every bolt-on and full exhaust. we both drive some highway and city. just hwy it's close to 23-25mpg.
It made her dad cranky.
On the way home I was driving 80mph and they followed behind us and when we stopped for fuel, my G/F's dad said he'd fill my tank and I was to fill his, b/c he said I must be cheating and not filling mine all the way up. This time the Stang was just under $8 bucks cheaper.