In a fixed – gear bicycle, without a free wheel, the rear wheel drives the pedals around. It happens when the driver shifts the gear into neutral or presses the clutch. Some Saabs and even some other cars have featured ‘one – way’ clutches which produce this effect by disconnecting the drive from the engine as soon as the engine over – runs; the engine can drive the wheels but the wheels can never drive the engine. This method saves fuel (though it eliminates useful engine braking) by reducing aerodynamic drag from the engine and gearbox. A freewheel or an overrunning clutch is a device in which a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft takes place and this how it is defined in mechanical or automotive engineering. Sometimes an overdrive is mistakenly called a free wheel though it is otherwise unrelated. A condition like the free wheel exists in the automotive area with a manual transmission going downhill or any situation wherever the driver has to take his or her foot off the gas pedal which closes the throttle then the wheels want to drive the engine, possibly at a higher RPM. In a two – stroke engine this could be a catastrophic situation: as many two stroke engines are dependent on a fuel/ oil mixture for lubrication, a shortage of fuel to the engine would result in a shortage of oil in the cylinders, and the pistons would stop moving after a very short time thus causing extensive engine damage. Saab also used a free wheel system in their two – stroke models for this reason and maintained it in the Saab 96 V4 and till early Saab 99 to achieve better fuel efficiency.