The record of 845.3 miles (1,360 km) was achieved during a roundtrip tour of Southern California on August 23 and 24, 2021. The car consumed 5.65 kg of hydrogen.
"The official record attempt was closely monitored by Guinness World Records, adhering to its strict rules and documentation procedures. On its efficiency-focused August 23 and 24, 2021 journey, the Mirai logged an impressive 152 MPGe with water as its only emission. GUINNESS WORLD RECORDS adjudicator, Michael Empric, validated the Mirai’s tank with a seal at both the beginning and end of the journey."
That's more than twice the EPA range (402 miles in the case of XLE version and 357 miles in the case of Limited version) and significantly more than the European record of 1,003 km (623 miles) and still had 9 km of range remaining set in May 2021.
The other record set by a hydrogen fuel cell car belongs to the Hyundai NEXO - 778 km (484 miles) in 2019.
According to the press release, the record was achieved by drivers skilled in hypermiling techniques that optimized the Mirai’s vehicle performance under specific weather and driving conditions.
"Driven by professional hypermiler, Wayne Gerdes and co-piloted by Bob Winger, the two-day trip began on Monday, August 23, 2021, at the Toyota Technical Center (TTC) in Gardena, California, home of Toyota’s fuel cell development group. The duo traveled south to San Ysidro and then north to Santa Barbara cruising through Santa Monica and Malibu beach along the Pacific Coast Highway. They returned to TTC that evening and logged 473 miles with only two driver swaps that day.
The next day, August 24, 2021, consisted of more local driving loops, where they pushed through 372 more miles of morning and afternoon rush hour traffic on the San Diego freeway between Los Angeles and Orange County until the Mirai had no more hydrogen left and coasted into TTC with a grand total of 845 miles driven, as witnessed by Empric."
"By the end of the trip, the Mirai consumed a total of 5.65kg of hydrogen and passed a total of 12 hydrogen stations along the drive routes without refueling. The Mirai was driven mainly during rush hour traffic in temperatures between 65 to 83 degrees Fahrenheit."