Bangladesh, Oct. 8 -- President Donald Trump has moved to confront one of the most politically explosive economic problems facing American farmers, truckers and consumers: painfully high diesel prices.

On October 5, Trump signed a new executive order designed to provide immediate relief by temporarily allowing tax-free dyed diesel - normally reserved for off-road equipment such as tractors, combines and other agricultural machinery - to be used by trucks and other vehicles on public highways. The measure took effect October 5 and remains in place through December 31.

The basic idea is remarkably simple: let people who desperately need diesel buy the same fuel they already use for agricultural and off-road operations, without immediately...