New evidence shows that ammonia emissions from road vehicles are underestimated in the UK national inventory. Remote sensing was used to measure ammonia emissions from over 230,000 vehicles on road under real driving conditions. Estimates of total ammonia emissions from gasoline passenger cars in the UK are around 2.6 times higher than reported in the national inventory, with emissions in urban areas underestimated by a factor of 17. Remote sensing data provides insight into factors affecting ammonia emissions like vehicle model year, mileage, manufacturer, and driving conditions like cold starts. More accurate vehicle emission estimates are needed to understand impacts on air quality and public health.