When I did plugs in the Subaru, I removed Autolites that I had put 30kmi on, and replaced them with the same part number. There was no
ScanGauge-discernable change in fuel consumption at idle, idle RPM, or fuel consumption in neutral at 2500 or 4000 rpm. I didn't bother to look for an impact on my fuel logs, since those go up and down all the time.
