I've got this same problem on my 850. I don't know, but I'm told that there is some kind of "overflow" valve in or near the gas tank and that when cars get old the seals don't work well anymore - therefore when you fill up all the way it leaks. Next time you fill up, look under the back of the car and see if it's leaking. Mine leaks like a waterfall if I fill it all the way so I usually look at the gauge, estimate how much will get it 90% full and stop there. I do plan on fixing it, the fumes are horrible and it's kind of scary to start your car up with a puddle of gas below.
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1995 850 Sedan, Naturally Aspirated, 143k mi
No major upgrades yet.