Car rental AVIS parking lot is full since there are no customers during COVID-19 pandemic at JFK airport. (Photo by Lev Radin/Pacific Press/LightRocket via Getty Images) Travel experts say the country is seeing a major car rental shortage as more vaccinated Americans hit the road and rental car companies have trouble keeping up with the demand.But experts said all isn’t lost for those who haven’t looked or are still searching for a vehicle."So the number one piece of advice if you need a rental car this summer is to book early as in book right now," AutoSlash founder Jonathan Weinberg told FOX Television Stations Tuesday.