It's a fact that running an engine with the ignition timing retarded from
its optimal point produces higher exhaust temperatures, as well as reducing
the power output. Therefore, if you retard the timing in order to avoid
pinging when using regular gasoline, then you will get the situation that
your author describes.
My own experience (on a fuel-injected Toyota Corolla GT-S) was that running
regular gas gave higher mileage than running premium, on back-to-back
tankfuls. In either case I flogged the car fairly vigorously. The car
includes knock sensors and computer controlled timing which should allow for
automatic optimization (to the knock limit) of poor gasoline usage. I don't
know offhand how long it would take the system to stabilize at a new timing
setting (especially when going from 'bad' gas to 'good'). This engine was
not designed for exclusive use of premium fuel, so it should have been a
fair comparison.
Theo
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