what causes mileage to drop? used to get 25 town, 33 highway, now only 18 mpg

This engine has a valve recession problem. Valve gets tighter and tighter until it doesn't close all the way, then it burns, you get misfire code, bad mileage, etc. See Car Talk: http://www.cartalk.com/blogs/tom-ray/today-whats-causing-ahmads-cr-v-stall