(Reuters) - Vehicle thefts in the United States recorded their biggest yearly drop in four decades as automakers, mainly Hyundai and Kia, bolstered anti-theft measures, while easing supply chain constraints made stealing vehicles for parts less lucrative.

Hyundai and Kia cars topped the list of the most stolen vehicles in 2021 and 2023 as their lack of immobilizers made them easy targets for stealing, fuelling viral social media trends such as the "Kia Boys".

Although cars made by Hyundai and Kia were among the top five models that were stolen last year, their thefts fell by about 37.5%, data from the National Insurance Crime Bureau showed.

Hyundai Motor America said it implemented measures to combat thefts of certain entry-level models lacking immobilizers by including free software upgrades, ignition cylinder protectors and reimbursement programs.

The automaker, whose Korean parent owns a third of Kia and shares platforms and components, said 68% of vulnerable vehicles had already received the anti-theft update.

Vehicle thefts across the U.S. fell 16.6% to 850,708 units in 2024 from a year earlier, according to the NICB data. They hit a record high of 1.02 million vehicles in 2023.

Improved supplies of auto parts and new vehicles have also made stealing cars less attractive for thieves.

Carjackings spiked during the pandemic as supply chain disruptions drove up prices for vehicles and parts, but the economic allure of vehicle stripping has since waned with better inventories, said Art Wheaton, professor at Cornell University.

Washington state witnessed a significant drop in vehicle thefts, while Nevada, Nebraska, Oregon and Colorado also saw a big fall in the number of stolen vehicles in 2024.

While models like the Ford F-150, Chevrolet Silverado, Toyota Camry and Honda Civic topped theft lists due to their large presence on the streets, four Hyundai and Kia vehicles in the top 10 for 2024 still point to broader issues, Wheaton said.

Meanwhile, Washington, DC, had the highest vehicle theft rate last year, with 842 thefts per 100,000 people, even though the U.S. capital saw an 18% drop from a year earlier.

California and New Mexico followed the capital region with theft rates of around 460 units for every 100,000 residents.

Alex Piquero, professor of criminology at the University of Miami, credited law enforcement efforts to disrupt theft rings, while the Council on Criminal Justice noted that fewer black-market sales, both domestically and internationally, may also have contributed to the decline.

(Reporting by Nathan Gomes and Akash Sriram in Bengaluru; Editing by Arpan Varghese and Anil D'Silva)