The vehicle was discovered in the area burned over by the Sand Fire. Lynne Tolmachoff, a spokeswoman for Cal Fire, said investigators have not yet traced the owner.
"A vehicle drove into the dry vegetation and ignited that," Tolmachoff said.
The fire was reported to be 50 percent contained late Sunday. About 1,500 firefighters were trying to hold the line.
Another fire in the Yosemite National Park area burned at least one home. The fire, first reported Saturday, grew to more than 2,000 acres Sunday.
Yosemite was still open to visitors, but evacuations were ordered in Foresta and El Portal.
"This fire has been fast moving and there has been incredible firefighting work done to protect employees, residents and the park," Yosemite Park officials said in a statement posted on Inciweb. "The fire is on both national park lands and Stanislaus National Forest."
The Sand Fire destroyed more than houses. The blaze burned cabins at the Rancho Cicada Retreat in Plymouth and gutted 13 antique cars on the property.
Temperatures topping 100 degrees made the firefighting effort more difficult. A prolonged drought has brought an early burst of wildfires to California, Oregon and Washington.