In Brazil, home of flex-fuel cars that can switch between gasoline and ethanol, prices for the biofuel are the most attractive they’ve been in years.

The ethanol-gasoline price ratio reached 65 percent last week at Sao Paulo gas-station pumps, according to oil regulator ANP. This ratio has the potential to reach 62 percent through May, something not seen since 2015, Martinho Seiiti Ono, president of biofuel-trader SCA Trading, said in a telephone interview.

“It will trigger a spectacular increase in ethanol demand and the constant decrease of the sugar mix,” Arnaldo Luiz Correa, director at Archer Consulting in Sao...