It's a 2002 E-320.  I ran about half a tank and made sure I stayed below
about 3500 RPM but occasionally by foot got heavy we went varrroommmm.
In town I got 18 mpg and 28 on the road.  If that's taking a hit I
wonder what it would be like with high test in the tank.  ;-)

Thanks for the reply...

Thanks friend, that's very complete and easy to understand.  I've saved
a copy so I can refer to it again.

T.G. I almost always agree with your advice but knocking, or pre-ignition,
occurs because the air/fuel mixture does not burn quick enough. A low-tech
but brief explanation of what happens is this. Premium fuel has a lower
flash point, thus burns more rapidly. When a lower octane fuel is introduced
into a cylinder under certain conditions the air/fuel mixture burns slower
and can pre-ignite.

As the piston moves upward compressing the mixture the spark plug ignites
the mixture right at the plug itself, this actually occurring before the
piston reaches TDC. As the flame front begins moving spherically from the
point of ignition the expanding gases begin driving the piston down in the
cylinder. However, another process is also taking place, namely the
compression of the remaining air/fuel mixture which has yet to ignite and
help continue the downward force on the piston.

If this remaining air/fuel mixture is not of adequate octane the mixture
simply explodes from compression alone, exactly like that which occurs in a
diesel engine. This explosion creates tremendous pressures and the
resounding knock associated with the phenomena.

Two things normally can be done to reduce this effect. The first is to
retard the timing so that the ignition occurs later in the cycle, thus the
piston has moved further down in the cylinder as the flame front expands,
reducing or completely eliminating the pre-ignition event. The second is, of
course, to increase the ability of the flame front to move more rapidly by
increasing the octane rating of the fuel. Premium fuel or the addition of
octane booster will solve this problem.

A couple of additional points. First, pre-ignition creates a lot of unwanted
pressure and temperature in the cylinder. This can result in the softer
piston material wearing away rapidly. If you ever tear down an engine that
has undergone pre-ignition for a long period of time you will always note
that the damage portion of the top of the piston is furthest from the spark
plug gap. Second, the heat buildup in the cylinder because of pre-ignition
sometimes causes deposits on the top of the piston to glow red hot and
continue to provide ignition after the key has been turned off. This is
often called dieseling.

This is a somewhat lengthy explanation. Hope it was more educational than
boring. Again T.G., not to dismiss what you say but perhaps expand on it.

I have to agree.  I saw a dramatic increas in milage when I started
using Premium (it is 93 octane here).  My dad had the car for a year
(300E) and ran it on regular.  He complained it was sluggish and sucked
gas. (actually, it was my stepmom who complained).  I found out she was
putting in regular gas, I changed it to premium, and viola!!  Better
mileage, better power.