Economy vs. speed

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Here's a question for the group. For years we have been told "the faster you drive, the more fuel you use," because drag increases on the
car etc. etc.
My wife and I own a '98 Z3 convertible, 2.8 litre w/a 5-speed manual transmission. We have two different driving styles. My wife gets a kick out of seeing how much mileage she can wring from a tank of fuel. She gently short shifts through the gears, monitors her speed constantly, and believes that a steady, high-gear 60 to 65 mph gets her the best milage of twenty six to twenty seven mpg. My driving style is exactly the opposite, and I squeeze every bit of performance out of it that I can. I go in fast, brake hard and shift at high revs.
We took a fairly long trip through Indiana last weekend, where the speed limit is 70 mph, and traffic was moving between eighty and 85, with a few stretches of 90 mph. Imagine our surprise when my wife flipped the computer read out to mpg, and it read 29.7 mpg! This really seems to fly in the face of what I have been taught about speed and fuel efficiency. Does anyone have an explanation?
BTW the top was down, and fuel type is always constant. We use Chevron Techron 93 octane exclusively, so that shouldn't be a factor.
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60~65 seems poor to me.
The OBC will generally not give you an accurate mpg readout for an entire tank, so it's best to compute gas mileage the old fashioned way - miles traveled divided by gallons used, averaged over a number of fillups.
FWIW, my '97 Z3 2.8 normally yielded around 29 mpg at 65-70 mph with a high of 32~33 for relaxed mountain driving (altitudes above 3,000 feet) at 50 - 60 mph.
Tom K.
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The OBC gives spot-on mpg readings vs. manual calculation of miles per gallon. Surprised me, but there it is.

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You didn't tell us what gear was engaged during most of the trip . Was it the 4th or the 5th ? There's a _big_ difference ! The economy doesn't come from the smoothness of the foot on the pedal but from the engaged gear hence the revving of the engine. It was true in the past with carburettors equipped with accelerating pumps. Each action on the pedal, if not controlled, was resulting in two coffee spoons of fuel injected in a pure loss !
I drive a Z3 coupé 3.0i, 5-speed manual gear box. I always use SP95 (95 RON). Didn't notice any difference with 98 RON. For daily commuting, I make 10 to 10.2 l/100 (~23 mpg) with 22 km on the highway @ 65 miles and 8 km in the city. Traffic lights are everywhere. Since mid 2004, the streets have been reduced from 3 lanes to 2 in order to provide more surface to the dogs <grin> Of course traffic jams drastically increased my consumption. Before these great improvements, it was around 8.8 / 9 l/100km.
Last year, back from holidays, I made an experience : Driving cool on mountain roads, using the highest gear as possible and engine braking, climbing and descending passes very smoothly, the DTE increased up to 1200 km !!! I ended up with 700 km before filling the tank (46 liters) ! ==> 35.75 mpg. I had to drive 50 km in traffic jams. I'll try again in the opposite direction.
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The car was in fifth gear, and it was a new tank of gas, so the counter was zeroed out at the start. We sometimes get a couple of tanks through the car at speed, but usually the tanks are mixed. A lot of the driving on a tank will be my wife's in traffic with a lot of lights going back and forth to work. The highest octane rating we can get here in our corner of the U.S. is 93. I would LOVE to try a tank or two of 98 in the car!
says...

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The car was in fifth gear, and it was a new tank of gas, so the counter was zeroed out at the start. We sometimes get a couple of tanks through the car at speed, but usually the tanks are mixed. A lot of the driving on a tank will be my wife's in traffic with a lot of lights going back and forth to work. The highest octane rating we can get here in our corner of the U.S. is 93. I would LOVE to try a tank or two of 98 in the car!
If you're using U.S. 93 which is (R+M)/2, it's equivalent to 98 Research octane (RON). There are basically three ways of measuring octane, with Europe using RON and the U.S. using an average of Research and Motor Octane.
Even if you could get U.S. 98 octane, it would probably just be a waste of money, as your car is optimized for around 91.
Tom K.
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Exactly so.
91 US = c. 95 Europe/international, optimum for the vast majority of cars. Even saw a note to that effect from Porsche about their cars the other day in a newspaper motoring section.
Petrol suppliers in Europe are now very careful about the promotional wording regarding their much dearer 98 octane fuels.
Loads of people either try to gain financial advantage (lower octane) and performance advantage (higher octane) and I think they are, to put it politely, either wasting their time and/or their money.
DAS
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It would be interesting to see the results of a controlled test of "regular" (87 U.S.) and "premium" (92~93 U.S.) for both horsepower and fuel economy for a BMW optimized for 91 octane.
My BMW R1200RT motorcycle manual recommends 98 (I assume that is RON), but indicates that it can be run on 91 with reduced performance. Buried in the manual is a spec for the "special fuel" horsepower (101) versus the usual claimed 110; so they are apparently saying regular gas gives you about 8% less power - at only a 5~6% cost savings.
Personally, I fill my BMWs with 93 U.S. if available.
Tom K.
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this might be true if the engine in said car was not equipped with a knock sensor; for engines that are so equipped, the higher the octane rating the better - for both performance and economy.
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The optimum economy and performance is achieved when following the manufacturer's recommendation.
DAS
For direct contact replace nospam with schmetterling
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And it's perfectly OK to smoke so long as you read the warning on the packet first.
What planet are you from?
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Equating tobacco companies' on-packet warnings with car manufacturers' fuel-octane recommendations?
Wacky 'baccy-induced non-sequitur?
DAS
For direct contact replace nospam with schmetterling
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Nah. You just carry on citing as the truth everything you read that a manufacturer writes on his product, there's a good lad. P. T. Barnum was right.
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A knock sensor just retards the ignition basically, so it depends what the maximum advance is set for octane wise. And I suspect it is UK 95 since this is the common one in Europe.
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this is true in part: oxygen sensors *should* pick up less volatile (higher octane) fuels, and lean out the mixture accordingly. this would lead - in the very worst scenario - to better gas mileage, if not increased performance.
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I've done some pretty long distance tests on 95 and 97/8 octane in the UK - motorway driving in the main with cruise in use over about 500 miles and can't say there's any difference in fuel consumption (528 auto) I'd also say if the petrol maker had firm evidence that the mileage was better - even on just one model of car - they'd use it in advertising supported by the test data.
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the way that increased octane produces a slower, more efficient burn at high compression temperatures would indicate that your tests were of the wrong kind, if you don't mind me saying: the benefit would be better shown by high-engine-speed driving ... probably of the kind associated with throwing a car around in urban situations.
having said which, i pretty much agree with the caveats about high octane fuels stated by the other posters; the difference is minimal ... but noticeable to those people who notice that kind of thing.

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Any MPG differences would be hard to prove under those conditions. Performance, yes. But again I couldn't truly say there was any.

It makes a difference up to the point where you reach the rating the engine was designed for, IMHO. Going above that is simply a waste of money. And all BMWs are designed for the Euro standard 95 - apart from some M models.
If there were a real difference in performance to be achieved by exceeding the 'normal' octane rating it would be easily proven by testing. And I've yet to find hard evidence of this being the case.
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totally agree. if you wouldn't mind me submitting a test sample of precisely one, here is my empirical experience:
daily drive from hull in e yorks to the A1/M62 roundabout, about 40 miles i think. cruise set at exactly 100mph both ways (outward trip at 6am, return at 9pm, so very light traffic both ways ... and obviously if there are any snarl-ups, i reduce speed accordingly). weeks 1 and 2 are using 95 octane, with a resultant MPG of 19.8 average. weeks 3 and 4 are using 98 super, with a resultant MPG of 22.2.
go figure. and yes, i do know that consistency is im;possible to establish in a real-world situation like this. but still, it makes me think.
and i also realise that the MPG differential probably does not pay for the price gap in the two octanes. just food for thought, is all.
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Huh? Exactly how does an oxygen sensor in the exhaust gas stream respond in any way to the octane of unburned gasoline?

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