97 bad mileage?

ive got a 97 suburban with the vortec 350, its got about 175,000 miles on it and its pretty much stock. lately its only been getting about 13-14 mpg. is that about average? it used to get about 15 but ive read a few people on here that are getting 20-21 mpg. what are you doing different? and do those tornado mileage improver things that go behind the air filter actual work?