Two entirely different thoughts.
First - supposedly she had gone out wearing pajama pants and a hoodie? If she was planning to go to a party or to meet someone her family wouldn't have approved of, she could have been wearing other clothes underneath. I'd like to know what (if anything) she was wearing when she was found.
Second - I avoid pumping gas, but in the past when I've done it, there have been occasions when I've gotten gas on my hands. If she did, she could have wiped her hands on her pants - or she could even have splashed gas on her clothes. If she smoked, I could envision a scenario where she was driving down the road, tried to light a cigarette, perhaps dropped it or the lighter - her clothes catch fire, she panics and loses control of the car and runs off the road. Hits her head and is stunned or knocked out. By the time she comes to, a small fire has become a big fire, and she's breathing in smoke and flames...
Some years ago, about a quarter mile from my house, a car had some sort of mechanical failure (I don't remember the details). Driver pulled over to the side of the road and got out, but the car ended up totally engulfed in flame, with the flames nearly reaching the roadside power lines. No idea whether oil/gas tank ruptured, but it was definitely a case of a huge fire starting without any outside accelerant being applied. So it can happen.