Costco Premium Gas in a BMW 645ci

Wife seems to think she is losing a little get up and go in her car.We wondering if it was the gas??I told her to use a premium brand for a week or two to see if that made a difference.We also thought that she is used to the car now and it just doesnt seem as quick as when she first purchased it.Any thoughts.?

Might be the gas, might not be. BMW’s for the last 15 years or so have computers that learn and adjust to the driving style of the owner. When the car is new it may feel more aggressive. IF your wife drives conservatively most of the time, and only notices the differance when looking for the extra power, that might be it. So try changing fuel, won’t cost much, check the air filter, can make a noticeable differance, and if nothing else, take it to the dealership and ask if they can re-flash the computer and tell your wife to drive the car harder. One other possibility. She may not have it in one of the sport modes of shifting(assuming automatic/SMG), this might make it seem like it is lagging. Good luck.

Unless BMW recommends or requires premium fuel, you will get no benefits from the higher octane. Or are you referring to the brand of gasoline, not the grade? In that case, it will also PROBABLY not make a difference either, as one gasoline supplier delivers gasoline to many different “brand” stations. However, the particular Costco station you patronize could have a contamination problem with its tanks or some other snafu, so try another station to see if it makes a difference.

There could also be some routine (and not-so-routine) maintenance items causing the problem. What is the maintenance history of the car? Is everything up to date?

“We also thought that she is used to the car now and it just doesnt seem as quick as when she first purchased it.”

You may have hit the nail on the head.

I would agree with your second thought, that it is merely a matter of her perception. If that is correct then the fuel would not make a difference.

However, I read into your post the notion that she normally purchases regular gas. If your owner’s manual recommends premium gas and she has been filling up with regular since Day 1, then you ought to switch permanently to premium from now on. One should not scrimp nickels and dimes on such a fine car.

Hey Bo, where you been hiding?, mostly ;-}

Got fed up with the lack of navigation controls on this board, but I just couldn’t stay away!

It’s good to have you back.

Wife always purchases premium gas.The Costco in question is brand new,so contamination is not an issue I would hope.She does not drive in the sport mode,I do though!!!I will have her ask her BMW service rep about the computer learning her driving tendencies and performing accordingly.Thanks folks!!

As others have said…if the car doesn’t NEED high-test then you don’t need it.

But I have seen lower performance when using Mobil/Exxon. I don’t know what it is, but it’s noticable. I wish it wasn’t because they are the MOST convenient to where I work and live. It’s NOT just on my 4runner…seen same results on the Pathfinder, wifes Accords and her new Lexus.


I’m pretty sure a 645i requires a minimum 92 octane fuel or so (near sea level). I use Costco gas for my two vehicles that require 92+ octane (Turbo Cayenne and SL55)and it works just fine. I can’t tell the difference between Costco and any other hi-octane brand. However, the more you drive a fast car the more it seems normal to be fast, it’s psychology I think. I have people get in my cars and gasp when I floor it, and I don’t think it seems so fast.

The 645ci appears to require 91 octane gasoline. If you have been using 89 or 87 octane, read you owner’s manual to verify whether 91 is required or recommended. If it is only recommended (I doubt it) the timing will be adjusted to make the lower octane gas detonate at the right time. That will definitely reduce the car’s power.