In a system according to one embodiment of the invention, an apparatus mounted onto a vehicle receives a control signal from a key. The control signal indicates one among a plurality of discrete and unitless vehicle performance levels. The vehicle performance (e.g. engine speed or fuel supply rate) is restricted according to the indicated vehicle performance level. Vehicle performance may also be restricted in a case where the key becomes separated from the apparatus (e.g. because the operator has become separated from the vehicle).