I got 24mpg on my 5.0 Cobra on the highway when it was stock. They come with 3.08s though so I was turning like 1800 rpm doing 75mph. They get suprisingly great mileage till you start modding...
As for 3.8 vs 5.0 it isn't even in the same performance universe.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.