speedo problems

I put a 4:10 gear in my car not long ago and it threw my speedo off by about 25 or 30 mph. I knew it would be off going from a 2.73, but dang! And on top of that the needle jumps up and down while driving and occasionally while stopped. And I was wondering how would I go about fixing these problems? Oh yeah I forgot to mention Ive got an AOD tranny if that matters any. Thanks