I would take it in for a full inspection. It could be many things causing to decrease gas mileage.
〉 Answered on Jun 3rd, 2014 by Gelina Aquilina, Service Advisor / Mechanic at
Prior to gas mileage decreasing, did you purchase new tires? Low rolling resistance tires are required for the mileage to stay where it should.
As the hybrid battery gets older, it will also decrease mpg.
Check the spark plugs. Worn spark plugs or spark plugs not correct for the application will lead to decrease gas mileage.
A good hybrid shop should be able to find the problem.
〉 Answered on Jun 3rd, 2014 by Helaine Kurot, Owner/Technician at 360 Automotive