Sudden drop of MPG in 2000 Altima GXE. Please help.

My wife's Altima GXE (2000) suddenly dropped MPG.
We bought this car few months ago with 56,000 miles on it.
MPG drop happened after I replaced old dirty air filter (which came
with the car) with the new one (bought at Strauss Auto). Air filter
change might be just a coincidence. The car used to make 300+ miles
per gas tank. Today the tank was half empty after just 90 miles.
Is it possible that the new filter p***** through too much air and air
flow meter instructs computer to inject too much fuel?
What else might contribute to such a drastical drop of MPG?
Service Engine light is not on, so I have no reason to suspect O2
sensor...
Thanks for all advises.