Dealers and automotive analysts agree that used vehicle prices are spiking across the country. A variety of factors, including the nation’s weak economic recovery, high gasoline prices and the March 11 earthquake and tsunami in Japan, have converged in recent weeks to send demand for used vehicles skyrocketing and supply plummeting. NADA projects that average June trade-in values for used cars will be up by 18 percent since December and by a full 30 percent over June of last year.