40 CFR §610.41

1. (a)In order to measure the effectiveness of a retrofit device at least two, and in some cases, three vehicle configurations defined in § 610.11 will be tested. Each vehicle will be tested at least twice in each configuration, as determined by the Administrator.
2. (b)The first test configuration is a baseline configuration. In this configuration the baseline or unretrofitted vehicle emissions will be measured.
3. (c)A second test configuration, an adjusted configuration, may be required at the discretion of the Administrator if a device requires both hardware and engine parameter modifications to achieve the fuel economy improvement. If, in the Administrator's judgment, based on a review of the available information, the combined effects of retrofit hardware installation and parametric adjustment could be substantially duplicated by parametric adjustment alone, then the Administrator may specify a second test, to evaluate such adjustment exclusive of the retrofit hardware.
4. (d)The third series of tests, in the retrofitted configuration, will evaluate the full retrofit system installed on the vehicle.