Adjust gas gauge?

[Breadstick]

After reading the "how many in-cab tanks" thread, I realized that my gas gauge has to be off because I filled it up from near empty the other day and only put about 9 gallons in it. On the bright side, I'm getting better gas mileage than I thought. However, I don't like the fact that I don't have accurate readings, which could be problematic on trips. Is there a way to adjust the gauge somehow to make it read correctly?

When I fill up the tank, the needle goes to the far right hash mark in on the gauge, but when I turn off the ignition the needle goes so far left that I can't see it in the viewing window. I figured that the far right mark was full and I would fill it up when the needle goes between the middle and left mark, but I'm starting to think that I'm off.