Hello. I had a fuel pump going bad in my 2.2 07 Cobalt. This caused problems, but my combined mileage was about 27-30 per gallon. I got a new fuel pump. My engine has more power, but my combined mileage dropped to 24-27. Is this normal?
Was the battery disconnected and maybe has reset the mileage counter? Also if the mileage counter was reset it usually is lower in colder temperatures. Mileage in warmer temperatures is usually higher. It also depends on idle time during cold weather. Personally I notice mileage way down because I’ll let the car warm up before I get in and go on cold days or nights. Not sure if that’s your case.