Nobody was killed in the incident, which occurred at about 3.30pm on Sunday in Orlando, Florida, and left a red Kia Sorento on the side of the road with its roof blown partially off.
Police say a husband and wife were driving the Kia, which had a propane grill inside that had the gas valve switched on.
The wife lit a cigarette as the car was driving near the entrance to the Central Florida Fairgrounds.
The resulting explosion blew out the windows of the car and peeled open the roof of the car.
The car traveled off the road and crashed into a pole, police said.
The man and woman suffered burns that were not life-threatening, and were transported to a hospital.
No further information about the couple was immediately available.