1997 camry sudden drop in mileage after changing oil pump
 

I recently installed a new OEM oil pump on my 1997 Camry due to a major oil leak at oil pump. After changing my mileage dropped from 27-28 mpg highway to 23mpg highway. After 3 tanks of gas I am now at about 25mpg highway. I drive 35 miles(highway) each way to work. May be my imagination but car seems to run rough above 80mph (I normally drive 80-90mph to work). I feel vibration above 80mph and power seems to be less. I pulled upper timing cover back off and checked valve timing. Looks right but difficult to be 100% sure it is not advanced a tooth. I really do not want to redo the timing belt again. Does anyone know if this could be the cause of my sudden bad mileage/loss of power? Any other ideas?

I think you should put back the old oil pump, I mean same brand or stuff but a new one.