Fuel efficiency is becoming very, very important, especially with a high-mileage driving situation. It makes sense to look for a more fuel efficient car that could also be cheaper to insure.
I think this family, given that they’re putting 1,800 miles a month on their car, should strongly consider making a change. Even if gas prices stick to their current levels, it ends up being a good deal for them over five or six years. If gas prices continue to go up, they’ll be in tremendously good shape in the future, especially if they couple it with some changes in habits.
My advice for them is to go for it. Once they get past the three or four year mark (depending on their payments), they’ll be in much better shape in terms of fuel efficiency. If they choose to drive the car for many years past that, then fuel efficiency will have saved them a lot of money.