Modern V8s usually have 4 O2 sensors, but only 2 upstream O2 sensors got involved in fuel injection and can affect gas mileage. 2 downsteam O2 sensors are only used to monitor Catalytic efficiency, they only bring up a check engin code/light when CAT efficiency is low.
27.4 mpg is quite impressive, the best gas mileage I got from my 92 400E was 9.5L/100km (24.7mpg) when highway cruising in summer, in winter, the best I can get is 12L/100km (19.5 mpg).
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.