from 35+mpg to less than 30mpg? from a new short ram air intake? help!

I am a proud owner of a 1989 Civic LX 1.5 with 205,XXX on the engine. I do not race or care about euro-tail lights. I do care about keeping my Honda in great shape and getting the best possible MPGs. I recently decided to pull the stock air filter and replace it with a cheap, short ram air intake to increase air flow, give a little bit more power, but most importantly, to get better mileage. I also replaced my valve cover gaskget due to a unrelated issue. Before my intake swap, I was getting 350-370 miles per tank. Now, I can barely get 300. On other forums, I've heard of reseting my ECU due to the increase of oxygen from my 3" intake pipe, apparently that would change the air/fuel mix. How would I exactly do that? Can I do that in my own garage? Others say I might need to pull the oxygen sensor. Please help me if you can! Thank you-