So far, I've had PS2s (OE sizes), RS3s (275s) and Z1 Star Specs (275S) on my M3. All returned about 18-19mpg on the highway. I put on some PSS and I'm getting 22mpg even when I'm doing 90mph (in an 85mph zone before the holyier than thou group start to admonish me for speeding and how I will kill innocent kids, insects, and animals and I am totally irresponsible and they have never even gone 1mph over the speed limit...I hate those people
).
Is that possible? I also took my car in for service for a bad vacuum brake booster sensor. But I've only gotten at best 19.5mph on the highway since I've owned the car.