Gas grade for '98 528i

I've got a lady friend who has one of these....she says the manual instructs her to use mid-grade gas, which obviously costs quite a bit at the pump
these days.
Will her using regular unleaded present much of a problem for her car? And if so, what would the problem (or problems) be? Performance degrade or worse?
Thanks.
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Basically, Higher octane runs hotter, thus the ignition keeps the carbon deposits down. If she plans on keeping the car for any amount of time, she should use at least 91 octane. Otherwise she may end up doing a head job in the future which usually runs around 6k. Your really not saving that much by going with the cheaper gas, its about .20 cents more that the good stuff, which on a 20 gallon tank (which the 528 is not, its less) is $4. If that $4 is going to make or break her bank account then she should get a Metro. Just my .02
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Corey Shuman wrote:

No it does not. It burns slower, not hotter.
-Fred W
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This question just got asked and responded to last week. My recommendation is, check your tire pressure every week and use the cruise control as much as you can when feasible. If you do this, this will provide fuel consumption savings that will offset the cost of higher priced fuel. The engine needs a high octane fuel to run properly within it's design spec. and running it out of spec. will probably lead to some failure down the road that could cost a lot more than the fuel savings achieved.
By using a lower octane fuel, you force the Engine Computer to adjust it's operation to maintain the design Spec. and thus you could end up with reduced distance travelled which means that you saved $5.00 at the pumps, but you burned the savings up by only travelling 300 km between fillups instead of 350km.

instructs
And
-
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Thanks...I did google before posting (goggling skills in question, obviously) and even scanned this group....but on the scan I guess my eye didn't catch the post made on 3/28. I found it after closer inspection, however.

Thanks much. I'll go read the other thread too.

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Roger Zoul wrote:

I've got a 97 528i with about 203000 miles, and I use the recommended grade which is 89. I do mostly highway driving and back road country driving. No stop and go city driving. I've used the higher octane once in a while, 91 or 92 I don't really recall, but I don't notice any difference in performance or gas mileage. Once my wife put in the lower octane, 87, and the check engine light came on. I don't know what code it was giving, but on the next fill up I changed back to the higher octane the check engine light went out and it's been running great ever since.
Joel
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In the UK, the lowest octane is 95, and I can't remember which US octane this approximates to as they're measured in a different way.
My 528 says 95 (UK) inside the filler. 97/8 octane is available and I can't be certain it gives ether better performance or economy, despite having tried reasonably controlled tests, like a long journey involving more than one tank of fuel and the extensive use of the cruise control. If I were to guess, I'd say about 33 mpg at an 80 cruise going up to 33.5 with 97 octane. But other factors over a journey of this length are just as likely to make it experimental error.
It would be interesting to conduct proper testing on a rolling road. I'd guess if it was easy to prove, the petrol makers would use such tests in advertising.
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Approx 90/91 octane US.
DAS
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Dave Plowman (News) wrote:

The only measure of gas mileage I use is what the built in computer indicates. I understand that this may not be the accurate, but it does give me a relative idea of how different driving styles affects gas mileage. When I go to visit my parents at the Jersey shore, which is about 3.5 hours, 170 miles, I set the cruise for most of the journey at about 70 to 75 mph, but I do have to slow down for tolls several times :( , I average about 32 to 33 mpg. In my normal daily driving I travel about 47 miles one way to work. About 15 miles is the country road driving, which includes a lot of up and down twisting winding roads. The remainder, about 32 miles, is open highway which I cruse at about 70 mph. I've gotten pulled over for speeding on the highway and it ain't worth the couple of minutes I save. If I use a heavy foot on the country roads my average is about 22 to 23 mpg, but if take it easier I have been averaging about 27 mpg. That's what about a 15% increase? Thats better than I can do at any bank around where I live :)
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When I first got my 528, I checked the mpg by running it near empty and brimming the tank, several times, noting how much, and logging the mileage. My results were the same as the computer. So I guess it's as accurate as the mileometer. Of course if that's as poor as the speedo, then it's not saying much. ;-)
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I made up an excel spreadsheet with all the formulas to calculate MPG and $ per mile. Now I just have to remember to get the receipt and write down the odometer reading.
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Get a logbook. I've had one in every car I've owned. I even carry one in my motorbike jacket 'cause my bike's odo resets at 100km. Some say that the new cars give better consumption on 95 unleaded than on 97 leaded. (That is in South Africa). I fear that none of these folks have the logbooks to backup their theories.
The problem is however, that you need to exactly meet the road conditions and driving style for your experiment. My wife can't get close to the fuel economy that I get, and that is just driving style. Good luck!
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your fuel is expensive!! try 86p a litre in the UK !!! that's over $7 a gallon in your terms

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Roger Zoul wrote:

Actually - it tells her to use 91 octane - which is premium, not mid-grade, unless you have a different rating system.

The car will run fine - the knock sensors will degrade the performance to achieve that... it will also get less miles per gallon which in many cases offsets the savings you get by using the lower octane.
It really IS one of those "if you have to ask.. you can't afford it" sort of things. This is a premium motor vehicle, it was $40,000 when new, so things like maintanence, fuel may cost a bit more. If she wants Ford costs - the best bet is to get a Ford.
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