Rarely does one have enough fuel economy sample cars to test for a high statistical confidence in the conclusions. The diagnostic inspection demonstration projects supported by NHTSA provided just such an opportunity.
Project data on fuel consumption collected from participants under actual day-to-day driving conditions showed a 4.7 percent improvement in fuel economy after repair to correct emissions. If this could be projected nationwide, it would mean saving about 1.8 billion gallons of gasoline per year, and a subsequent savings of $1.1 billion per year for the consumer.
In a concurrent study with EPA, samples of compact and intermediate cars were tested for emissions and fuel economy. Minimum repairs were made to meet inspection criteria, with an average repair cost of $16. Of these cars, 65 percent required no parts. This data indicates that periodic diagnostic emission inspection and maintenance of rejected cars can be administered at low cost, reduces pollution levels of HC and CO, and significantly improves fuel economy.
Analysis of the emissions data show an overall average reduction of 22 percent for hydrocarbons and 12 percent for carbon monoxide. The vehicle failure rate for emissions decreased from 23 percent to 10 percent over a one year period.