However, the test result only reflects the performance of the test trucks on a given day, with specific loads and drivers. That is, tests must be repeatable in order to be fully validated. Multiple tests, using different vehicles at various locations, can result in very precise mpg figures. Results typically are expressed in terms of the percentage of fuel saved, or the percentage of improvement. The key variable, however, is traffic volume.
Traffic causes air turbulence.
The denser the traffic, the greater the turbulence. This also can be accomplished on-road, in clean air. However, if you want to test in mountainous terrain, it's necessary to test on the road. Good test routes can be found anywhere in the United States. But avoid routes with traffic lights, weigh stations or rush-hour slowdowns.