well when i start at lvl 2 and i start to make my car i get to 8+ lvl by removing, repairing and installing parts
you just need to read under the hood or lockpick a lot of doors and start constructing your large car (something large like apc) removing and installing parts give enought levels to install everything except aditional engines but some grinding like creating a deathmobile or tank and later repairing your vehicles will help you with that