Commercial truck drivers drove 4% slower in April compared with the start of the year, a new analysis by INRIX, a transportation-analytics company, found.

The reason is likely because truckers are trying to save on gas. Average U.S. diesel prices were at $5.38 per gallon on Friday, and they had been at $5.66 per gallon in May, according to AAA. 

Slower speeds — even a few miles per hour — reduce drag and increases fuel efficiency, the Wall Street Journal reported. This can produce hundreds of dollars in savings for truckers every week. The same is true for passenger cars, but the INRIX analysis found no evidence cars are slowing down. 

The high fuel prices hit owner-operators especially hard as they typically cover their own expenses. 

The INRIX analysis tracked more than 60 million commercial-truck hauls across 10 major U.S. metropolitan areas in the first four months of the year. Besides slower speeds, the analysis also found a slight decline of 2% in the average length of trips. 

The complete analysis covered through April, but an initial analysis shows the trend toward slower speeds and shorter trips continued in May.