ProjectH213

so i bought a 91'240 sx se about 2-3 weeks ago. everything worked fine for about 84 miles on the trip and then it stopped working.... so i filled up my tank and reset the trip to check how many mpg i was getting and it stopped at 33 miles... what can be the problem? my speedometer and my tach are both working so i don't believe it has anything to do with the sensors... i'm not really complaining since its not adding any miles to instrument cluster, but i would like to beable to know how many miles i get to a full tank of gas and how many miles i've gone since my last oil change... any help would be greatly appreciated. thanks in advance..