Two other boys who drank the mixture at a party last week were hospitalized, treated and released, said Dr. Donna Seger, executive director of the Tennessee Poison Center.
The Greenbrier Police Department identified the deceased as Logan Stephenson, 16, and J.D. Byram. Stephenson died last Thursday and Byram died Monday.
"This was an unfortunate accident," Seger said. The doctor told CNN the four boys were in a group that purposely mixed the soda with methanol (instead of ethanol).
Methanol can make people feel inebriated, but a very small amount of methanol can be toxic, Seger said.
The two boys who died drank far more than the others, she said.
Seger is worried because "dewshine" is not a new concoction. While people should be familiar with the dangers, it is unclear whether this incident is an indicator of a larger problem.
"Typically, people don't come up with names for something if it's an isolated event," she said. "We need to educate people."
The doctor said her staff has checked with other poison centers but hasn't seen any other cases.
The medical examiner's office has not yet received the results of toxicology tests. Police and the Robertson County Sheriff's Office are investigating the boys' deaths.
Some racing fuels are a methanol blend and some are 100 percent methanol. Methanol also is used as a solvent.
Methanol can be purchased at specialty stores such as motorcycle or boating shops.
The Centers for Disease Control and Prevention says methanol's toxicity comes from its metabolic products.
"The byproducts of methanol metabolism cause an accumulation of acid in the blood (metabolic acidosis), blindness, and death," the CDC says on its website
Methanol poising can cause confusion, dizziness, drowsiness, headaches and the inability to coordinate muscle movements. It can prompt nausea, vomiting and heart or breathing failures.
Mountain Dew, a subsidiary of Pepsi, makes and markets a soda called DEWshine