The range was downgrounded after the car maker discovered an external test agency hadn't followed correct procedures for the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).
The 39kWh Kona Electric has been recalculated with a 180-mile range, down from 186, while the range of the higher-spec 64kWh variant has been reduced from 292 miles to 279.
Meanwhile, the 39kWh e-Niro is rated at 179 miles rather than the original 193 miles, and the 64kWh model has a range of 282 miles rather than 201 miles.
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”.