I have recently had a loss in gas milage, 270 to 225 per tank. My Service Engine Light has come on for the EGR. I took it to Saturn and they said the O2 sensor and EGR valve are working properly. They did a fuel economy test that said I was getting 32 miles per gallon. So how does real life conflict with the computer? How can I be getting approximately 20-22mpg when their computers are saying my car is running fine and getting great gas mileage? What else could be wrong?

I have recently had a loss in gas milage, 270 to 225 per tank. My Service Engine Light has come on for the EGR. I took it to Saturn and they said the O2 sensor and EGR valve are working properly. They did a fuel economy test that said I was getting 32 miles per gallon. So how does real life conflict with the computer? How can I be getting approximately 20-22mpg when thier computers are saying my car is running fine and getting great gas mileage? What else could be wrong?
The onboard computer is only a estimated calculation.

Physically documented fuel consumption and mileage indicated by the odometer should be used to determine actual mileage.