Computer mileage vs. calculated mileage has been discussed on here before. I track mine on my phone with an app called Gas Cubby and I'm about 6% lower then the car computer. (27.4 vs 29.1).
I had wondered about this before and wondered if it could be that the car computer only tracks the gas usage when you are actually moving, and does not include the data when you are stopped but idling. Since the instantaneuos milage info goes to "---" when you are stopped, there is no data to record. Its overall milage number is a cumulation of the instantaneous data. In that case it would have you using less gas (not recording the usage during idle) for the same miles. When you do a calculation, you are basing it on all the gas you use, but over the same milage.
I had wondered about this before and wondered if it could be that the car computer only tracks the gas usage when you are actually moving, and does not include the data when you are stopped but idling. Since the instantaneuos milage info goes to "---" when you are stopped, there is no data to record. Its overall milage number is a cumulation of the instantaneous data. In that case it would have you using less gas (not recording the usage during idle) for the same miles. When you do a calculation, you are basing it on all the gas you use, but over the same milage.