BMW feels good when the belly is full?

I usually wait till the tank gets near empty so that I can reduce the number of trips to the gas station. I pull in to the gas station, fill 'er up and get back on the road. Pretty mundane story, except that the
car drives smoother with a full tank. Now, it wasn't riding rough before I filled her up and I can't 'feel' the deterioration as the tank gets progressively empty if my life depended on it, but I have been feeling this for the last 5+ years.
Maybe the feeling is purely psychological as I feel 'ready to hit the road' with a full belly. But none of the other cars I have driven...
Curious to know if someone has had a similar experience. BMW thang? My fuel pump?
2000 323i 4Dr US
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I have a 99' 528i and have noticed this also, When I get down to just about 1/4 tank left, the car starts to Idle a bit rougher and not work "As well" but once I fill the tank up to the top. everything is A-ok once again for another 3/4 of a tank..
-Branden

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I'm confused. Can you or can you not feel the difference in a full or empty gas tank? You said, "the car drives smoother with a full tank," then you said you can't "feel" the deterioration as the tank gets empty if your life depended on it, then go on to say that you've been feeling it for the last 5+ years.
Either you feel the change or you don't. If you don't, what's the question? If you do, what's the question?

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Further, when was the last time you changed your fuel filter?
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Sounds like you're by yourself in the confused department. Apparently, the others who responded did not have problems comprehending the question.
I can feel the 'smoothness' coming on when the tank goes from <5% full to a 100% full. But as I put miles on the car and it drops from say 35% to 30%, I can't feel the 'roughness' coming on probably because it is gradual. The question is "Do you know what I'm taking about?".
J Strickland wrote:

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Probably due to the fact that your suspension has a slight load with a full tank. Probably eliminate the "roughness" with a good 4 wheel alignment and wheel balance.
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I guessed that was your complaint, but I didn't want to say that the fuel load would affect the balance of the car because the fuel load always affects the balance of the car, but most people are not atuned to feel the changes.
Yes, I think I understand your question, and the answer is that the gas has weight, and when the weight goes away, the suspension seems to become more harsh. I think this is a normal condition.

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Also, you didn't say, but have the shocks been changed out? If so, the new shocks may not behave to the changing weight distribution as the fuel tank empties.
Your gas tank should be under the rear seat, forward of the rear axle. It should affect the front as well as the rear, but it will obviously have greater impact on the rear.
In any case, I suspect you are noticing a normal condition. Relax and enjoy your car.

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Vuestra Merced wrote:

Weight bias (F/R)? Smoother with more weight on the suspension? Psychological effect of just having parted with $50 (better do something good!)?
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-Fred W
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Whilst BMW's have a better front rear balance than FWD cars, the unladen weight distribution is still front heavy (where the engine is). If you fill up you put quite a bit of weight low down and centred over the rear axle (on most BMW's anyway) and the front / rear balance of the car improves.
The other factor is that when the tank is less than full the fuel sloshes about, making handling less precise as the fuel shifts from side to side when cornering.
So well observed, but no cigar.
OTOH you will use more gas if you run filled up.
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