speedometer problem

Today i put a set of new tires on my 94 cavilier and fdrov to school and it was fine. when i was driving home i was behind an old lady, she was doin 20 in 1 35, after about a minutes my speedometer went all the way down to "0". and my gas guage went up to 7/8 full when it was at 3/4 when i filled it the prvios day. please help me why does the spedometer not work, is there a problem in the engine or the instamint panel?