Declining fuel efficiency on 2005 Lincoln Aviator?

I bought this vehicle in May and at that point the sensor reading for MPG was 14.7. It has gradually decreased to now reading 13.0....... Is this a normal rate of decline and anyone know what may be causing such a sharp decline? Seems quite precipitous to me. 15,000 miles @ purchase, currently has 20,300 miles. Thanks for any help you may be able to provide.