In business, the twins looked exactly alike and dressed the same. They even trimmed their beards the same. People they worked with said the only way to tell them apart was each had a different reaction when you told him a joke. As other car companies moved from steam to gasoline power, their Stanley Steamer became the most popular steam powered car. As the internal combustion engine designs improved, however, steam and electric cars lost much of their popularity. Finally, the numbers of cars sold went down to less than 600 a year in 1918. That year the Stanley brothers sold the company to Prescott Warren, a Chicago businessman. both brothers retired. A few months later, while he was driving home from a trip to Maine, Francis' car ran off the road. Francis was driving a new 1918 Model 735 Stanley Steamer. He died in the accident, July 21, 1918.