[A/S] Counter Intuitive Fuel Cost Saving Tip

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

If you have a recent vintage, gasoline engine powered tow vehicle with a
computer controlled engine, try PREMIUM fuel.  It might be less costly per
mile than cheaper regular gasoline!

I know that the common knowledge is to use the fuel (octane) grade
recommended by the manufacturer in the owner's manual.  The common belief is
that using "Premium" (high octane) fuel in an engine "designed" to run on
regular grade gasoline is a waste of money.

This logic was correct for older model engines which had the "basic ignition
timing" set by bolting the distributor to the engine in the propper
position.  The only spark advances were vacuum and mechanical (centrifugal).

However, most modern engines (within the last few years) use computerized
engine management systems which dynamically control the spark advance to the
current, instantaneous, operating conditions.  They dynamically advance the
spark (ignition timing) as much as possible without causing pre-ignition
(ping).  These engines have a "knock sensor" which tells the computer
whether it can advance the spark, or must retard it. The computer
continuously tries to maximize the spark advance as much as possible.

Note that optimum spark advance will optimize the thermodynamic efficiency
of the engine and get the most heat out of the fuel in the combustion
chamber.  Speed of burning is increased by lighter, more volatile fuel
(lower octane), humidity (burns slower when the air is damp and there is
more moisture in the air/fuel charge), and load on the engine (increases the
likelihood of pinging).  Thus the old ignition timing settings were a
compromise at best.

Since higher octane fuel is slower burning and (in the case of unleaded
fuels), has more heat units per volume, it can produce more power if burned
optimally (by firing the spark plug earlier (advance) to get the full amount
of heat (and combustion pressure) out of the fuel.

Thus, you may be able to get slightly better fuel economy from premium fuel
over regular grade - enough to more than make up for the higher price of
premium.  Plus, the engine will respond better.  Note that as the price of
fuel goes up, the percentage increase in the fuel price of premium over
regular goes down (assuming the price differential remains the same.

In my case, my 2003, 6.1 liter Silverado gets 10.5 to 11 mpg towing with
premium, vs. 9.5 to 10 with regular.  This is approximately 10% improvement
in fuel economy, yet a $ .20 to $ .25 higher cost for premium vs. say $2.80
for regular is less than 10% more.  Thus the fuel cost per mile is less for
premium than regular!

The only way to know for sure is to try it and see.

I would be very much interested in your experience.

Good luck,

Oliver Filippi