Peugeot paid particular attention to aerodynamics in the design of the 308 to improve the overall performance of the vehicle. As a result, the car has a low drag resistance (SCx 0.67 and Cx 0.29), resulting in a reduction in fuel consumption and CO₂ emissions.

The 308 will be available with a choice of six lean burn engines, including three gasoline engines (1.4-liter VTi, 1.6-liter VTi and 1.6-liter THP) developed as part of the partnership with the BMW Group, and three HDi diesel engines (two 1.6-liter units and one 2.0-liter), one of which will have an emission level of 120 g CO₂/km in the combined cycle.

Gasoline engines. Both VTi engines are equipped with a VVT (Variable Valve Timing or continuously variable timing) on each camshaft and with a variable lift system on the inlet valves developed from the BMW Group’s VALVETRONIC concept. The variable valve lift system adjusts the valves’ maximum stroke gradually according to the position of the accelerator pedal.

The 88 kW (118 hp) 1.6-liter VTi provides the 308 with a good dynamic performance while reducing fuel consumption to 6.7 l/100km (35 mpg US) in the combined cycle—a decrease of more than 10% compared to a 307 equipped with the old 1.6 liter 110 hp engine. Maximum torque is 160 Nm (118 lb-ft) at 4,250 rpm, with 140 Nm (103 lb-ft) available at 2,000 rpm.

The 1.4 liter VTi offers 70 kW (94 hp) and 136 Nm (100 lb-ft) of torque at 6,000 rpm, with 117 Nm (86 lb-ft) at 2,000 rpm. Apart from the smaller capacity resulting from a shorter stroke, this engine features all of the technology found on the 1.6 VTi engine, including the dual VVT system and the variable inlet valve lift system.

The higher performance 1.6 liter THP (“Turbo High Pressure”), features a twin-scroll turbocharger and produces maximum power of 110 kW (148 hp) at 5,800 rpm (103 kW at 6000 rpm in the automatic transmission version) and maximum torque of 240 Nm (177 lb-ft) available at 1,400 rpm.  Fuel consumption is 7.1 l/100km (33 mpg US).

Diesel engines. The 2.0-liter diesel develops maximum power of 100 kW (134 hp) at 4,000 rpm and  torque of 320 Nm (236 lb-ft) at 2,000 rpm. An overboost function activated during hard acceleration can push the torque up to 340 Nm (251 lb-ft).

The 2.0-liter engine features a variable geometry turbocharger; an electronically controlled high pressure injection pump (allowing a maximum pressure of 1,650 bar in the common rail); and piezoelectric injectors with six apertures, resulting in optimal injection both in terms of quantity and duration. Fuel consumption is 5.5 l/100km (43 mpg US) in the combined cycle.

The 80 kW (107 hp) 1.6 liter HDi diesel develops torque of 240 Nm (177 lb-ft) from 1,750 rpm.  It also features a variable geometry turbocharger and its high-pressure direct injection system (up to 1,600 bar). The engine offers fuel consumption of 4.7 l/100km (50 mpg US)—0.2 liters less than a 307 with an equivalent engine.

The 66 kW (88.5 hp) 1.6 liter HDi develops maximum torque of 215 Nm (159 lb-ft) at 1,750 rpm. Derived from the 1.6 liter 80 kW engine, it is equipped with a fixed geometry turbocharger and uses the same direct injection technology. Fuel consumption is 4.5 l/100km (52 mpg US) with CO₂ emissions of 120 g/km.

Transmissions. The 308 offers combinations of 4-, 5- and 6-speed gearboxes. A 6-speed electronically controlled manual gearbox—a new offer within the Peugeot range—is coupled with the 80 kW 1.6-liter HDi diesel. This 6-speed controlled manual gearbox eliminates the need for a clutch pedal.