Here's the scenario
The plugs in the E320 were Bosch Supers.
I changed them to Bosch Platinum 4s.
Fuel mileage dropped from 29mpg to 26mpg.
One hypotheseis is that the Platinum 4 plugs require more voltage
and so were desaturating the coil. As a consequence, not enough
spark for a good burn.
Another is that they were too efficient, prompting the computer to
input more fuel, thus increasing consumption.
(Last night I changed back to the Supers.)
What are your thoughts on this?