I don't think LE would actually say she WAS driving the truck that day if she wasn't. She was driving it.
To the original poster's point, asking about frequency of Mustang to Truck, it is a good question.
We know the Mustang is Terri's car. That's for sure. And the couple drove her car to the gym that day when they scurried away as the journalist asked them what the public could do to help them find their little boy. They both got into the car very quickly and shut the doors. Terri got into the driver's seat.
All of this, right there on TV, for everyone to see that night on local news. And record and rewind and watch again. Awkward, to say the least.
I think Terri probably drove her car to all of her volunteering at Skyline, too. But on Science Fair day, LE has said she was driving the truck. That vehicle would have made her BLEND IN that day, not only due to its neutral white truckness, but also because she is normally associated with and noticed in her RED GT MUSTANG with custom plates.
Did you really need a double cab truck to carry a poster board and diorama to a 2nd grader's science fair? Let me just leave it at that.