very bad gas mileage

I had a recent battery problem(the negative terminal end was very corrored probably because of water getting on it and wasnt getting any conduction with the battery). I replaced the negative terminal end with a new one and after getting this replaced ive been noticing a large reduction in my fuel economy. I dont know exact numbers, but on a 10 mile trip the fuel guage dropped about 1/10, which is horrible compared with how it used to be. Anybody have any ideas why or how to resolve his? My guess is that disconnecting the battery caused all the sendors to reset and now they have to readjust, but its been driven for about 30 miles already and i dont think it should take this long for them to adjust.