Fuel

TDI engines operate on diesel fuel (also known as petrodiesel), or B5, B20, or B99 biodiesel subject to manufacturers' prior approval.
In fuel efficiency, and clean emissions when run on biodiesel, or when converted vegetable oil (which should NOT be used on the later PD engines without prior conversion, since irreparable damage will result)^{[citation needed]}, TDI engines are among the best on the market.^{[citation needed]} A 2007 Volkswagen Jetta Mk5 with a 1.9 TDI engine and a five-speed manual transmission, for example, achieves 5.2 litres per 100 kilometres (54 mpg_-imp; 45 mpg_-US) on the European combined-cycle test, while a six-speed Direct-Shift Gearbox (DSG) automatic version reaches 5.9 litres per 100 kilometres (48 mpg_-imp; 40 mpg_-US).
Newer TDI engines, with higher injection pressures, are less forgiving about poor-quality fuel than their 1980s ancestors. Volkswagen Group's warranty does not cover damage due to bad fuel (diesel or bio), and has in the past recommended that only mixtures up to 5% biodiesel (B5) be used. Volkswagen Group has recently permitted mixes up to B20, and has recommended B5 be used in place of 100% petroleum-based diesel because of biodiesel's improved lubricating properties.
In North America, No. 2 diesel fuel is recommended, since it has a higher cetane number than No. 1 fuel, and has lower viscosity (better ability to flow) than heavier fuel oils. Some owners in North America, where cetane levels are generally poor (as low as 40), use additives, or premium diesel, to get cetane numbers closer to the standard levels found in the European market (at least 51) where the engine is designed. Improved cetane reduces emissions while improving performance, and may increase fuel economy.
New ultra low-sulfur petroleum-only diesel recipes cause seals to shrink and can cause fuel pump failures in TDI engines; biodiesel blends are reported to prevent that failure.