FedEx's buildup of its Ground network has raised more questions about its Express network, where growth has stalled. FedEx continues to pour money into new aircraft and expanding air hubs in places such as Indianapolis and Memphis.
Mr. Miller, the Memphis hedge-fund manager, said FedEx should consider slowing some of its capital expenditures until Express performance stabilizes.
FedEx executives say the upgrades are needed because the new planes are more efficient and reliable than older ones. The company has trimmed some spending plans and offered a buyout program to Express employees, and could cut spending further if the economy worsens.
The company has resisted calls to integrate its delivery networks. Drivers for Express and Ground operations pick up and drop off packages separately, sometimes arriving at the same place within minutes of one another, according to former FedEx employees.
Mr. Smith said keeping the operations separate is vital to maintaining the promise of speedy delivery by Express. If a snowstorm delays an arriving flight, he explained, Ground drivers can start their routes while Express carriers can wait for time-sensitive deliveries such as essential medical supplies.
This month, the World Trade Organization forecast that global merchandise trade volume would increase by 1.2% this year, down from the 2.6% growth it forecast in April. Over the past year, Mr. Smith has often appeared in Washington to make his case for defusing the U.S.-China standoff, which is at the core of global trade tensions.
Mr. Smith wasn't planning to be as involved as he currently is. The FedEx board changed its retirement rules to let Mr. Smith stay on after he turned 75 in August. His heir apparent, a longtime lieutenant, abruptly retired in February. Mr. Smith declines to say how long he plans to stay CEO.
"I'm very focused on the here and now," Mr. Smith said. "I don't care about any legacy. The legacy will be the success of the company."
Write to Paul Ziobro at Paul.Ziobro@wsj.com