In the end it all comes down to what 08WhtX5 said....."This is a very subjective question and strictly depends on your needs and desires. If cost isn't a factor then I would go with the 4.8."

I thought both engines required premium. Could I be using reg. or mid. grade in my 3.0?

The short answer: No.

The technical answer: not the best for your motor. It'll take it, but won't like it. Would you feed an athlete only canned foods?

Off Topic Response
 

Well, the premium being referred to is a price premium, not premium fuel.

To balance out AZNm's post:

The short answer: Yes

The technical answer: It will not hurt your engine. It MAY cause reduced power, and increased fuel consumption, depending on the actual fuel quality in your area, and your ambient conditions. The only way to know is to try it. If you notice any deterioration in power or fuel consumption, it is a pretty good bet that the ECM is retarding the timing to prevent predetonation; in that case, you should go back to premium fuel and get all the performance you paid for. I used mid-grade (89) for years with absolutely no adverse effects, but did notice a power drop on 87. That said, you won't save much in annual fuel costs, and many will slag you with analogies such as tinned food for athletes, don't buy a BMW if you can't afford the fuel, it isn't a Honda, etc. ;)

So I guess you don't want to be a 20% better global citizen? :rofl:

I don't see why it always comes back to fuel price. Fuel is dirt cheap compared to world prices. We need to get past the price issue.

My decision process was to buy a new 3.0 E53, not for the fuel cost, but because it used 20% less fuel than the 4.4 at the time. Improved handling and a smoother engine was icing on the cake. When we replaced it, we bought an X3 3.0si, and cut fuel consumption by another 15% in the city, 20% on the highway. Added bonus: the X3 is a second faster to 60 than the X5 with the same engine, and the 400kg we left in the showroom means the vehicle is that much more nimble. No, it isn't an X5, but it does what we want to do. I needed a six passenger vehicle in December, for a day, and rented a Durango with a 3rd row seat. Worked perfectly.

This is just one vehicle. But compared to a 4.8, we are up to 40% better on fuel consumption, in round numbers. What if everyone did that?

/eco mode off

I have to disagree with the 2nd part. As discussed many times before, over time you are doing damage to the engine. Engine will retard timing WHEN knock sensors say there IS detonation. Even when continuous monitoring, you still are getting the initial knock first for anything to happen. Over time, with detonation, you will have issues. Plain and simple.

The $$ that you save between premium vs mid or low grade is really little. ~10-20 cents a gallon. and say your fill up is 20 gallons. Well that's only $2-4 dollars a fill up. *52 weeks and that is only $104-208 dollars a year. If that's a problem, you shouldn't be buying a X5.

Point well taken. I do use premium.

Continuing Off Topic....
 

I fully respect your opinion. Mine is just different. The knock sensors are measuring vibrations representative of pre-detonation, on every firing cycle, per cylinder. It is an active feedback. Any active control system is going to require a feedback loop. It all depends on the threshold setting, how severe the abnormal combusion is (or if it ever even reaches the knock point). It would be reasonable to detect pre-detonation prior to any severe knock condition). BMW does state in their literature that lower octane fuel will not damage the engine. They also rely on the sensors for other than low AKI fuel: they are there to protect the engine from fuel problems, exhaust problems, carbon build up, and other things that cause abnormal combustion. They have to have designed in a threshold level.

As to the $$ that you save, I agree. In my area, it is $0.12 per litre, 80 litres per week, about $10 per weekly fill. $500 per year, and that is for regular. I used 89, for half that saving. (Note: I don't use 89 on the twin turbo 3.0, it is illogical) For the E53, it was completely irrelevant in the economic big picture. For many, it is easier just to pay it and not think about it. For others, there is no option because they require 91 or similar from their local pumps or experience power and mileage loss, due to their local fuel quality.

Good stuff Jeff. More things to consider beside knocking... these built-in thresholds can only accomodate so much. Typically with cheaper grade fuel, pretonation is only the tip of the iceberg. You still need to consider the ill effects/after effects of the carbon buildup from carbon deposits on your valvetrain, etc. Over time considering this and the fact the valvetronic motors with the high compression ratios, that $100-200 you save will end up costing you a much prettier penny.

Also the degradation will get more severe causing other things such as further retarding of the timing. You can only go such much BTDC before throwing CEL's, etc.

The manual on page 213 reads,

"Super Premium Gasoline/AKI 91; This gasoline is highly recommended. However, you may also use gasoline with less AKI. The minimum AKI Rating is 87. If you use gasoline with this minimum AKI Rating, the engine may produce knocking sounds when starting at high outside temperatures. THIS HAS NO AFFECT ON THE ENGINE LIFE."

Mind you, not every country in the world, (BMWs are driven there too) including South Korea, will not have three little buttons letting you choose the "different quality" gasoline at the pumps. They only have one nozzle for Gas and another for diesel and thats it!

It would not make any sense at all to specifically make the engine suitable for the 91 octane fuel as BMWs are driven internationally. (rumor has it that some countries even utilize gasoline with higher than 91 AKI rating for vehicular usage, ie close to jet fuel).

However, when you go to fill up your X5, there is that sign that says, "Premium gas ONLY" which helps to makes life extremely...simple?! lol

but really, to get into the business end of the whole premium gas deal and the cleaning agents and what not,, its highly an ethical issue than a scientific one.