I had a tune up done, replaced the Mass Air.

I had a tune up done, replaced the Mass Air Flow Sensor, After this work my car is getting 60 miles less per tank than just before the tune up. I replaced the Catalytic Converter because I was told this was the problem. I am still only getting about 250 per tank instead of the usual 300. I bought this car new and I have never gotten less than 300 per tank. What could be the problem.