I don't think that you can calibrate the guage. The reading is x-amount when whatever corresponding increase/decrease in reistance from the sending unit.

Just because I think doesn't mean jack flit! I'd like to see what other's say. I'll take a best guess and say that you may be able to calibrate the arm on the sending unit. Still, hold off and see if anyone has any other suggestions.

One more thing, check the sending unit's ground. Sending units are one thing that can have what you could call half ground, meaning that the sending unit's ground can have enough resistance to register, yet the reistance is enough for an incorrect reading.