Book excerpt: Studies show that cars use significantly more fuel per km than suggested by official certification test ratings, and some argue that this gap is growing as a percentage of the tested value. This has raised concerns that national fuel efficiency and carbon dioxide emissions reduction goals will not be met, and that consumers will lose faith in reported fuel economy figures. This publication analyses the fuel efficiency gap and examines technologies available that could reduce that gap and improve fuel economy, as well as considering policy options for encouraging uptake of these technologies by vehicle manufacturers and, in some cases, by consumers themselves.