In Plainfield, Ind., a bank cut the interest on car loans, and auto sales were multiplied by five. All Americans might join President Reagan in lauding this bank and others that have taken the initiative to stimulate local economies in such ways. Since banks have to pay interest on their money as well as receive interest when they rent it out, they cannot go on indefinitely if the spread is too small. But someone has to make a start somewhere if all interest rates are ever to reach reasonable levels. Go, Plainfield!