For several years the gauge in my 1999 95 would do a dance after filling the tank. The gauge would initially register full and then drop to empty and the warning light would come on. Then it would bounce back to full and then to empty, etc. This would happen on and off until the tank reached some where around half full. At that point the gauge would read properly. I brought this to the Saab dealers attention and was told that the baffles in the fuel tank can cause that to happen. He removed the baffles and that solved the dancing problem. However, the gauge now seems to read improperly. When I fill the tank when it is regestering empty I can only put <15 gallons in. I believe that tank holds 20 gallons. Yesterday I filled the tank and drove 75 freeway miles. When I ended the trip the gauge was reading between full and 3/4. I think that is to much of a drop for so few miles. To be certain that the issue wasn't just a poor milage issue I had the dealer do a tuneup and check all calibrations. Everythig checked out fine. The car justed turned 50,000 miles. Any suggestions?

 

Ever run it to completely empty? That would be the most accurate thing I can think of: run it till it stalls, top it off and see how much really fits in the tank and than run it till it stalls again and calculate the MPG figure and compare it to your SID figure.