Quote Originally Posted by MetroMPG View Post
I'm taking the car on a ~120 km round trip today with the wheel/tire combo from Mirage #2 installed (which caused zero ECM freakouts, and which appeared to be OK last night in a brief test drive). If it's uneventful, I'll swap the smaller rear tires back on when I get home and see if that triggers it again. That's where I'm putting my money.
That should be a good test of the different tire size combinations. Prior to ABS & traction/stability control being added to pretty every car these days, all this probably wouldn't have been a problem in the past.

What you are doing would be a big no-no on an AWD vehicle like my Forester. Running different size tire front & back wouldn't create drivetrain issues on a FWD car, but it may wreak havoc with sensors.

If you don't have problems today, but they reappear when you go back to the 165/65r14 tires you will most likely have your answer. I would suspect large difference in tire size is the problem, but I am just guessing, too.