While I am skeptical of the claims of 50% fuel mileage increases, I understand that piston engine airplanes are regularly run in a lean of peak EGT condition at cruise for the purpose of increased efficiency (no unburned fuel being used to cool the engine as is done in typical rich of peak operation). I suppose that a piggyback computer that induces lean of peak operation in an automobile could in theory cause measurable mileage increases. However, aircraft that are run at lean of peak typically utilize individual cylinder head temperature probes to ensure that one of the cylinders is not running so warm as to damage a valve. I would be leery of installing such a device on my own vehicle without an iron-clad warranty against damage.
Megginson Technologies: Land and Hold Short » Blog Archive » Leaning the Mixture