The official range of the Hyundai Kona Electric has dropped from 292 miles (469 km) to 279 miles (449 km) in its most powerful iteration, as a result of erroneous test data.

The range was downgraded after the car maker discovered an external test agency hadn't followed correct procedures for the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).

The announcement by Hyundai follows news from sibling brand Kia on Friday, in which it confirmed that its electric e-Niro had also had its official range decreased for the same reason.

The 39kWh Kona Electric has been recalculated with a 180-mile  (289 km) range, down from 186 (299 km), while the range of the higher-spec 64kWh variant has been reduced from 292 miles (469 km) to 279 (449 km). 

Meanwhile, the 39kWh e-Niro is rated at 179 miles (288 km) rather than the original 193 miles (310 km) , and the 64kWh model has a range of 282 miles (453 km) rather than 201 miles (323 km).

Hyundai said: “In testing the Kona Electric to establish its homologated electric vehicle driving range, the independent organisation overseeing the process accidentally provided an incorrect testing methodology and then approved the results it generated. 

“This led to the Kona Electric being tested for a disproportionate length of time on the WLTP ‘urban’ cycle – comprising lower overall vehicle speeds and a reduced energy requirement – resulting in an overestimation of the vehicle’s all-electric range.”

Hyundai added that the inconsistency in the test cycles “has been identified due to Hyundai’s ongoing homologation work” and said that the situation is being investigated to “arrive at a full explanation”.