For the first time, 13-liter engines have become the number one choice of U.S. Class 8 truck customers, according to R.L. Polk year-to-date engine registrations.

The top seller of 13-liter engines in the United States is the Volvo Group, which includes Mack Trucks and Volvo Trucks. The Mack MP7 engine is also the top-selling 11-liter engine in the U.S., according to Polk registrations.

“We’re seeing a shift in the market to less than 15-liter engines as customers increasingly focus on operational efficiency,” said Kevin Flaherty, Mack senior vice president for U.S. and Canada.