Sometimes when I go past 70mph my speedometer starts going crazy and says im going 120mph and says i have no gas and its completely cool. basically the dashboard is reading irregularly.

Also, one time my car wouldn't start so I tried jump starting it didn't work either, changed the battery then it worked without even jump starting it. A week later, Im already having the same problems so I take it to the mechanic in my area and he said he tested the alernator and it failed and that I should also get a new battery cuz the alertnator is the one that charges the battery....and at this point since my alternator is messed up that my battery is low as well even though i changed it recently.

at this point before i change my alternator, sometimes my car will start, sometimes it won't and what i do now when it doesn't start is i hit the battery with a closed fist where the cables connect and also shake the battery and power gradually comes back.

Let me know if anyone can suggest anything.......do i need to change just the alternator, or do i change both the alternator and battery, etc.

 

getthe battery load tested, and if you replaced it recently if it fails a load test it should be warrantied. if it dosent fail the test then it is a good battery, as far as "sometimes it won't and what i do now when it doesn't start is i hit the battery with a closed fist where the cables connect and also shake the battery and power gradually comes back." , maybe you should loosen off the connections and clean the heck out of them and then put them back on and tighten the snot out of them.