How much of a cheapskate are you?
Run it on the proper stuff or buy a Nissan, fer christs sake.
Don't even start on fuel prices, how much is it for 93 octane? $4.55 per
US gallon? I'm running an old '89 325i in the UK and feeding it proper
fuel even though it's costing me $8.99 per US gallon.
Your two cars are worth a lot more than my old E30, so why are you even
thinking of running them on cheap fuel? If you can't afford to fuel them
properly, sell them. You may as well. Running a good car on rubbish fuel
is like ordering a filet mignon in the finest restaraunt and then
drowning it in cheap burger relish.
If burning 87 octane in your car, when 91 octane is specified, will not harm
the engine, and the performance degradation is not noticeable in typical
driving, how much money can you save? The Energy Information Administration,
U.S. Government Department of Energy, offers some figures for US gasoline
retail prices (these are averages, all areas, all formulations). A year ago
Regular was going for $2.982 a gallon and Premium was commanding $3.196 a
gallon. The 21.4 cent difference delivered a 6.7 percent saving over
Premium. This June 23, 2008, Regular extracted $4.079 from your wallet while
Premium sucked up $4.312 for every gallon. The differential (23.3 cents) has
grown slightly since 2007 but buying Regular is now only 5.4 percent cheaper
than Premium. Since 5 percent is roughly the typical percentage of mileage
decrease to be expected with the 87 octane fuel in a 91 octane engine, is
there any savings at all?
This does not take into account possible future engine damage:
Thomas Plucinsky, BMW Product and Technology Communications Manager told us
all BMW engines are designed to run on 91 octane. All performance testing,
including EPA emissions and fuel mileage, is done with 91 octane. However,
though BMW is all about performance, their motors will run on 89 or 87
octane without damage. The knock sensors pull the ignition timing back and
eliminate detonation. There will be a loss of power and a decrease in fuel
mileage, but the size of the horsepower loss and the increase in fuel
consumption depends upon many factors, such as ambient temperature, exact
formulation of the fuel and driving technique, so BMW does not offer any
estimates for operation on lower grade fuels. One not so obvious concern,
Mr. Plucinsky noted, is the type and quality of additives the gasoline
companies include in the fuel. Premium gasolines may have better additive
packages which are more effective keeping fuel systems (particularly
injectors) clean and working efficiently, than those in regular grade fuels
or off-brand products. Using lower octane or off-brand fuel could be
degrading the fuel system over time, setting you up for a repair bill down
That's all well and good, but the OP asked about using 89 instead of 93,
where there is an actual requiremnt of 91.
Bottom line, he can use 89 if he wants, but his cost per mile will remain
pretty much the same. 89 costs less but delivers fewer mpg. Not a lot fewer,
just enough fewer that the cost per mile will remain pretty much the same.
89 will possibly produce a lower cost per mile, _maybe_. I use 89 instead of
91 and find that it saves me about $1.00 on a tank full of gas. It ain't
much, but it's a buck. I bothered a few times to calculate the cost per mile
of mid-grade and of premium fuel, and with the cost per gallon difference of
a dime, I found that the cost per mile was nearly identical, with the 89
octane gas getting a slight edge. I have to run the test again to verify my
earlier findings, but I recall the savings to be about $1.00 on a tank. The
cost per mile to drive was not perfectly offset, and the small margin went
to the lower grade fuel.
Since I'm dollar-driven, I go for the lower grade and accept the cost as
being less performance.
There is no danger of damage by using 89 instead of 91.
The cost differential is insignificant too, except that 0.4% is a growing
number as the price of gas goes up. Granted, the savings does not cover my
beer tab at dinner with my wife, I have found no reason to not use 89
instead of 91.
My Jeep has an aftermarket fuel injection system. And when I installed it,
the fuel requirement went from Regular (87) to Premium (91). It does not
tolerate 87 at all, and balks at 89, so I have to use 91 to avoid engine
damage. I know about the potential for big trouble resulting from using the
wrong fuel, but my BMW is happy to run 89.
I guess you didn't read my post.
"One not so obvious concern, Mr. Plucinsky noted, is the
type and quality of additives the gasoline companies include in the
fuel. Premium gasolines may have better additive packages which are more
effective keeping fuel systems (particularly injectors) clean and
working efficiently, than those in regular grade fuels or off-brand
products. Using lower octane or off-brand fuel could be degrading the
fuel system over time, setting you up for a repair bill down the line."
Gas going up has nothing to do with it. It is the price ratio of 89 octane
to 91 octane. In my
area that ratio is 4.33/4.53 or .95. According to the department of energy
(see my earlier post please) using regular gas
will result in a mpg reduction of 5%. So essentially buying regular gas
will result in a savings of 4.5% at the pump
but will result in a 5% reduction in mileage. All the mile increasing the
chance of a mechanical failure down the line.
Sounds like a deal to me!!!!!
Actually, a decreasing number. When gas was under $2, ($1.799, 1.899 and
1.999 say) there was a 10% savings going to 87 from 91/93, 5% with 89. Now
it's $4, $4.15 and $4.30 or thereabouts? You do the math.
I've been unable to find anything other than anecdotal information regarding
power/mileage losses with the lower octane fuels. For most of us, it would
be almost impossible to create an experiment that would be controlled enough
to offer empirical data that would compare the fuel strategy costs
If you're pinching pennies to operate your Bimmer, there are ways that are
equally or more effective than doing it at the pump.
R / John
And if you get around to the north side of the beltway, there are a few
suburban MD stations now pricing at 3.909, 3.979 and 3.999 for 87, 89 and 93
respectively - although their usual is a 10 cent differential between
For their R1200RT motorcycles BMW actually publishes a spec of 110 hp using
93 octane and 101 hp using 87.
Check the Dinan site:
" The E46 M3 is an excellent example of an engine that is sensitive to
octane. It has a very high volumetric efficiency as well as a very high
static compression ratio of 11.5 - 1. The engine being tested was a stock M3
engine. It was first warmed up and stabilized using the method described
previously for the M5, running 91-octane fuel. As you can see in Figure 22,
where the engine was warmed up and the previously discussed testing
procedures applied, the stock M3 produced 280 hp (Figure 22- violet line).
We then replaced the 91-octane fuel with 93 (available in most parts of the
country). The M3's computer was so quick to determine that the fuel had been
improved that it only took four dyno runs for the timing to adapt to the
increased octane and raise the power up to 291 hp (Figure 22 - light blue
and yellow lines). A gain of 11 hp with just 2 points of octane. "
It turns out that my car actually has 89 as the spec, not 91. I feed it 89
because that's what it wants. I was thinking I fed it 89 because it
But you guys are all correct, I'm saving at most $1.50 per tank on the fill
up, and could actually have a higher cost per mile that can eat the buck and
a half over 300-ish miles. (Assuming a price difference of a dime between
the cost of one grade and the next higher grade, and a fill up of 15
I just did some math, and was shocked.
Using my earlier assumption of a dime difference in the price of one grade
and the next higher grade, I did some calculations.
If the higher grade costs $4.00, then the lower grade will cost $3.90. If
the car takes 15 gallons to fill, and goes 300 miles, then the full tank
costs $60 and $58.50, respectively.
If the higher price fuel gives a range of only 308 miles as opposed to 300
miles, the extra cost will pay for itself.
Using the figures I pulled out of my ass, the $4.00 gas costs 20 cents per
mile, but the lower grade for the same distance only costs 19.5 cents per
mile. 15 gallons @ 4.00 costs $60, @ $3.90 it costs $58.50. If I divide 5850
by 300, I get 19.50 (or, $0.1950) per mile. In order to get the same cost
per mile for the more costly fill up, I only need to go 308 miles, that's
only 20.5 MPG. ONE HALF MILE PER GALLON is all I need to recover the added
dime per gallon for the higher grade.
In other words, if the proper grade gives more than one-half MPG better
mileage, it pays for itself.
I just want to say that I have run my E28 on sub-85 octane Mexican gas,
and while it retarded the spark like you wouldn't believe and had a tiny
fraction of the normal acceleration, it ran smoothly and did not ping.
I was impressed as hell... it's a pretty spectacular fuel injection system,
especially considering how little compute power they had to work with inside
that little 8-bit ECU.
"C'est un Nagra. C'est suisse, et tres, tres precis."
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