The A6 TDI engines give further evidence of just how closely Audi has been able to align sporty performance and efficiency. Fuel is delivered by the extremely sophisticated and quiet system of common rails with rapid-action piezo injectors and an injection pressure of 1800 bar (26,106.79 psi). The two V6 TDI engines (2.7 TDI with 140 kW (190 hp) and 3.0 TDI with 176 kW (240 hp) have peak torques of 380 Nm (280.27 lb-ft) and 450 Nm (331.90 lb-ft), respectively.

At the lower end of the diesel program is the four-cylinder 2.0 TDI engine, which is available in two versions: the 125 kW (170 hp), expected in fall 2008, and the 100 kW (136 hp). The basic version is an especially efficient e-model known as the 2.0 TDIe, which uses a six-speed manual shift or automatic multitronic transmission to deliver 320 Nm (236.02 lb-ft) of torque to the front wheels. With multitronic it sprints from 0 to 100 km/h (62.14 mph) in 9.9 seconds, and continues on to a top speed of 204 km/h (126.76 mph). Yet its average fuel consumption with a manual shift gearbox is only 5.3 liters per 100 km (44.38 US mpg), equivalent to CO2 emissions of app. 139 grams per kilometer (224 grams per mile).