The caller wishing to drive from WA to ME to visit, and return, in an old Subaru should give more consideration of flying.
Google Maps says Seattle to Portland is 3148 miles. At 25 MPG and $2.50/gallon she is facing $315 for fuel, one way, not counting overnight stays, wear and tear on the car and its tires.
Priceline.com has many options under $600 for the round trip.
Would cost less to fly and would save 6 days of travel. So unless one wishes to “See The USA”, flying is the only good option.