Really Poor Gas Mileage

Replace the air filter, thats a biggy, the fuel gauge bouncing is from the sending unit in the gas tank. Having the fuel pump replaced they must have replaced the sending unit. After market ones do this. Annoying I know.

How many miles on it? It may be time to replace the upstream O2 sensors. Is the check engine light lit?

Check the catalytic converter in case it is partially plugged up.

-I checked the air filter, It is an older K&N and was pretty dirty so I replaced it with an old (but much cleaner) regular air filter. There was an acorn, peanut, and some dried up tomatoes in the housing but no other signs of mice. I will see if that helps. I am pretty sure they replaced the sending unit as well, so that may explain why the fuel gauge is moving around. -The car has a little over 107,000 miles on it, with no check engine lights on. How would I be able to tell if it is the upstream O2 sensors that are bad? -The vehicle is not having any problems with power. Could it still be a partially plugged Catalytic Converter? Is there a way to test if it is plugged?

So an update, back in February my SES engine came on as well. I did not notice a performance issue, but my mpg were still down. I took it to the shop and they checked both o2 sensors and they were fine, they also checked the catalytic converter and it was plugged. They replaced the catalytic converter and now my mpg is back up to a little over 30 mpg. The only lingering question is (and it probably deals with the new fuel sending unit they put in when they put in the new fuel pump) So I use to get close to 400 miles per tank, now i only get 300 miles and the low gas light comes on. My calculated MPG shows that I should still have plenty of fuel (1/4 tank) when the gauge is showing empty. I really don't want to chance it and end up running out of gas on my 30+ mile drive to work.