We investigate technological change with regard to co2 emissions by passenger cars, using a free disposal hull methodology to estimate technological frontiers. We have a sample of cars available in the uk market in the period 2000–2007. Our results show that the rates of technological change (frontier movement) and diffusion (distance to frontier at the car brand level) differ substantially between segments of the car market. We conclude that successful policies should be aimed at the diffusion of best-practice technology, and take account of the different potential for further progress between different segments of the market (e.g. Diesel vs. Gasoline engines and small vs. Large engines).