[JamesM3M5]

You can only go so lean before the engine exhibits lean surge or lean misfires. The best way to increase fuel economy is reduction in power required to move the vehicle through the air and over the pavement. Tires are available with low rolling resistance ratings. Narrower is better, pressures in the 38psi range. I would caution against tuning leaner for better fuel economy, optimize in other areas. Or if you want speed and economy, get a 335d.