If you have an R1200GS you're probably quite annoyed at the time it takes to fill it up and you know you never get a really full tank.
If you read this article:
You'll find you can change that in ten seconds. I have tried this and it works, stick your finger in the hole and take out that beasty litlle rubber neck..
It makes quite a difference.
NOTE: There is a second modification there that you may be interested in doing but I find that just removing the rubber neck made a hell of a difference.