The USPS said earlier this month that the move, set to start Friday, would cut costs and increase reliability.
The new service standards mean USPS will increase the delivery time for 39% of first-class mail and 93% of periodicals such as magazines.
USPS will increase transit time by one to two days for first-class mail traveling longer distances. That means mail sent out of state will likely take five days instead of three days.
About 61% of first-class mail will remain at its current delivery standard, and 70% will still arrive in three days or less, according to a notice by the USPS in the federal register. In general, the delays will affect pieces of mail that have to go farther, from coast to coast or far reaches of the U.S.
USPS said if it would take you more than a day to drive your mail to its destination, it is likely going to take the extra two days to get delivered, so plan ahead.
Washington Post analysis found states west of the Rocky Mountains, plus Florida and Southern Texas are likely to be most affected. The Post found 70% of first-class mail sent to Nevada and 60% sent to Florida is likely to be delayed. Arizona, Montana, Oregon and Washington may see more than half of their first-class mail deliveries slow down.
The slower timeframe allows USPS to shift more of its delivery service away from air transportation and deliver more mail via ground transportation. The agency said airmail is more susceptible to unexpected weather delays and costs more, so the new system will be cheaper and more reliable.
Mail sent locally will continue to take a total of two days.
The details of the 10-year plan, developed after the backlog of USPS deliveries around the holidays, were first made public in March.
The Associated Press contributed to this report.