I keep a gas purchase log for all of my vehicles. Over the years I've
done the premium/regular comparison many times. I have never been able
to detect any significant difference in fuel economy. I've had several
vehicles with knock sensor and the claim was that if you used premium,
the PCM would adjust the engine parameters such that you would obtain
slightly better fuel economy and slightly more power. I have no way to
evaluate the power because the increase was relatively minor (3% to
5% - hard to detect without measuring equipment), but fuel economy
changes should show up over a long period of time. I've never seen
them. I know they should be there, but their is no way they can be
more than a few percentage points. I suppose if you have a car getting
25 mpg, a 5% increase would be detectable (26 mpg), but you need
careful records to determine this.
Logically, I can't see why you would expect a large increase in fuel
economy by switching to premium fuel - assuming the engine runs ok on
regular. Most PCMs only vary the ignition timing to adjust for fuel
type. While this does affect mileage, the effect at part throttle is
small. For conservative drivers, the difference is probably miniscule,
since even when running regular, they rarely stress the engine enough
to force the PCM to retard the timing. Without changing the
compression ration, I think the chances of measuring 6% or greater
differences in fuel economy are nil. With premium costing around 7%
more than regular, I think buying premium because of a possible
increase in fuel economy is bad economics. On the other hand, if you
are using your vehicle to haul a heavy load, towing a heavy load, or
driving at sustained high speeds, premium might be worth the extra
cost IF you have a car that can adjust the engine parameters (and most
modern cars can - although to varying degrees). And of course if you
have a car where the manual says premium is required, that is what you
From http://www.usatoday.com/money/autos/2003-07-30-premiumgas_x.htm :
"No data show that engines designed strictly for regular run better or
longer on premium.
"The Federal Trade Commission, in a consumer notice, emphasizes: "(I)n
most cases, using a higher-octane gasoline than your owner's manual
recommends offers absolutely no benefit. It won't make your car
perform better, go faster, get better mileage or run cleaner."
"There is "no way of taking advantage of premium in a regular-grade
car," says Furey.
"There is no gain. You're wasting money," insists Jim Blenkarn, in
charge of powertrains at Nissan in the USA.
"No customer should ever be deluded into thinking there's any value in
buying a higher grade of octane than we specify," says Toyota's Paul
Williamsen, technical expert and trainer."
"21. Will premium gasoline give better fuel economy than regular?
Will one brand of gasoline give better mileage than another?
Gasolines with higher heating values give better fuel economy.
Differences can exist, but they will be small compared to the benefits
to be derived from the maintenance and driving tips in the above
Traditionally, premium has had a slightly higher heating value than
regular, and, thus, provided slightly better fuel economy. The
difference - less than 1 percent better - is not large enough to
offset premium's higher cost. The difference is likely to be less or
nonexistent between grades of reformulated gasoline.
There can be differences in heating value between batches of gasoline
from the same refinery or between brands of gasoline from different
refineries because of compositional differences. The differences are
small and there is no practical way for the consumer to identify the
gasoline with a higher heating value.