Abstract
Wind tunnel tests, full scale coast down tests and extended road running have been compared as methods of evaluating the effectiveness of devices for reducing the aerodynamic drag of heavy commercial vehicles. Wind tunnel testing is recommended as the best method of examining basic aerodynamic effects. For the transport operator the best method is probably a road test comparing the performance of a modified vehicle and a similar unmodified one; in the USA a procedure has been recommended by RCCC/SAE. Coast down tests are not thought to be of very wide application; they are too susceptible to the effect of small variations in experimental conditions. (Author/publisher)