Lower mileage after oil and battery change

Hello, what are you doing to calculate fuel economy? You can't count on the on board computer to be 100% accurate. For best results, measure the amount of fuel consumed by filling fuel at a local station, resetting the odometer, driving and then refilling fuel at the same station to the same first click of the pump. Divide the miles into gallons pumped and you'll get a rough idea. When you change the battery, it is likely that any computer driven data may be reset for the short term and not be so accurate.