[MetroMPG]

The big loud exhaust probably also reduces low end torque (according to people who know about exhaust). Since that's where you're doing most of your economy minded driving, that's not good.

Also, the aftermarket wheels/tires may have more rotational weight near the outside, which isn't going to help in stop/go driving either.

Have you got a ScanGauge? Reason I ask is that more than anything, driving technique is how to save fuel particularly in urban driving. & feedback is priceless.

Welcome to the site!