Final answer:
The United States reduced speed limits to fifty-five miles per hour during the 1973 oil crisis as part of an effort to conserve oil following an embargo by OPEC and peaking domestic production.
Step-by-step explanation:
The country that dramatically reduced speed limits because of efforts to conserve oil was the United States. During the 1973 oil crisis, driven by an OPEC embargo and the realizations of peaking domestic oil production, the U.S. government took several measures to reduce oil consumption. One of these measures was the implementation of a national speed limit of fifty-five miles per hour on interstate highways. This action was part of a wider series of responses to an energy crisis that saw fuel prices quadruple, gas shortages, and the exploration of fuel-efficient technologies by automobile manufacturers. The crisis served as a wake-up call, highlighting the nation's vulnerability due to its heavy dependence on foreign oil and leading to changes in both consumer behavior and energy policy.