Almost two-thirds of consumers shopping for a new car or truck said rising prices are causing them to consider used models, according to a national study released this week at the 1995 National Automobile Dealers Assn. convention in Dallas. The survey, conducted by Glendale automotive marketing research firm Dohring Co., shows 62.1% of respondents said the high prices of new vehicles are prompting them to consider less-expensive used vehicles. The data underscores increasing concerns that rising vehicle prices are pushing more consumers into the used market. The NADA released a study this week showing that the average price of a new car will probably pass the $20,000 mark this year. A new car cost about $18,200 in 1993, according to the group, up from $17,100 in 1992 and $10,725 in 1983.