1999 Subaru Legacy excessive fuel consumption

The Last 3 tanks of gas, I have experienced a sudden 20% drop in mpg from 25, 26 sown to 20, 21 mpg with my 99 Subaru Legacy SUS, 2.5. This is my commuter and I cannot think of a single change in my driving habits. I have not noticed any fuel odor, the air filter is new (double checked anyway), no noticeable loss of power, same gas octane and station as always, EGR valve appears to be ok (it moves when performing the check), checked timing at warmed up idle (about 18 deg. Btdc at 750 rpm), tire pressure fine, can't find any vacuum leaks (although I haven't done all the checks with a vacuum pump), runs, starts, performs normally except for high fuel consumption. What would cause this sudden change?