My 1985 Euro 500SEC has a O2 sensor and catalytic coverter, but no #3 on the X11. Is there a way to hook up a #3 to check lambda? Maybe other tuning tips.

I am trying to improve my 15 MPG freeway efficiency. Car passed state emissions tests and most significant change was a new O2 sensor, but also leaned out with the 3mm allen wrench to almost stall, and changed timing from 0 TPD to 5 BTDC. (note Euro manual is 15 BTDC with higher octane.)

Kurt