man of steel

I noticed the past 3-4 months that my 1999 Avalon's mpg has gone down. I used to get an average of about 22-21 mpg. But now I'm getting 17-18 mpg. I only have 84,000 miles on this car. I noticed my tires are getting low on tread. I change my oil regularly, but I did recently change my oil to synthetic-blend high mileage vehicle from regular oil. Maybe my car is just too old. Has anyone experienced something similar?