However, analysts expect automakers to recoup lost sales and GM has said some sales would shift to the current quarter due to the hack.
CDK said on Tuesday it was running ahead of schedule for bringing the dealer management system back online, with most dealer connections already live.
Ford also said hybrid and electric models helped drive growth in the quarter.
Quarterly sales of its gas-powered models fell 5%, while EV and hybrid vehicle sales were up about 61% and 55%, respectively.
Overall, US new vehicle sales in June stood at around 1.32 million units, representing a seasonally adjusted annual rate of 15.29m units, according to data released by Wards Intelligence on Tuesday.
Reporting by Nathan Gomes in Bengaluru; Editing by Devika Syamnath.
-Reuters