Biofuels are mainly used mixed with fossil fuels. They are also used as additives. The largest biodiesel consumer is the U.S. Army. Most light vehicles on the road today in the US can run on blends of up to 10% ethanol, and motor vehicle manufacturers already produce vehicles designed to run on much higher ethanol blends. The demand for bioethanol fuel in the United States was stimulated by the discovery in the late 90s that methyl tertiary butyl ether (MTBE), an oxygenate additive in gasoline, was contaminating groundwater. Cellulosic biofuels are under development, to avoid upward pressure on food prices and land use changes that would be expected to result from a major increase in use of food biofuels.