wtf is up with my mpg?!?!?!

[lsmalley]

I have a 1990 190E 2.6L and I think that my fuel economy is declining, but I can't be sure because I have nothing to compare it too. My tank is about 14.5 gal and on a full tank in city driving I get about 280 miles till it runs down to R and the light comes on......But on the other hand, if I am on the highway I notice that if I reset the miles to 0 and drive on a full tank after about 50 or 60 miles the needle is just approaching the 3/4 mark. What could be the problem? and what does it mean for a car to "run rich?" I just picked that term up looking through the forum. Does the check engine light being on have anything to do with it? If I disconnect my battery for a min or 2 the CE light goes off and will stay off untill I turn on my A/C. Is that a separate issue?