[MetroMPG]

Given that it's an automatic, I think your best hope is DWL in top gear with the torque converter locked. Without instrumentation, DWL is harder to do. You'd be surprised at what the slightest - almost imperceptible - changes in throttle position do to your current fuel consumption.

For DWL, a vacuum gauge would work fine.

Or wire a multimeter to your MAP sensor and aim for voltage readings that correspond to lightest load.