Some simple Q's from a newcomer

[eltren]

Just picked a '99, it's all new to me. Overall, it seems pretty damn good.

Engine performance is decidedly underwhelming. I think I hear an exhaust leak.
The computer is telling me I'm getting 16 MPG. At 60 in 5th.

Is there a well known exhaust leak problem spot or component?
I don't notice any smells indicating an extreme rich condition.
Can the computer need calibration?

So to state the obvious, the car needs a tune, and...what?

You tell me.

thanks