Speedo fast and jumps at 70mph

[JRANGER]

So My speedo was crazy jumpy so I replaced the cable and it works great! Now I have tires that are 2 big so the speedo reads about 3 -5 mph faster then I actually go but when I hit 70MPH it starts jumping between 70 - 100mph...

So what could cause this?

Is it the speedo gear? If so do I need a smaller or bigger tooth gear to get it closer to the right speed?

Currently have a 21 gear speedo gear

Thanks

[beamn7]

Found this online awhile back, should help you get pretty close. (drive gear is gear that is on trans output shaft. Pull the speedo gear adapter out, mark one tooth on the drive gear with a marker, jack the rear end up and with the trans in neutral, rotate the driveshaft and count the number of teeth you have.
As for the jumping, might try lubing the speed cable. Disconnect at speedo head and disconnect at trans. Spray lube into the cable at the speedo head connection and let it run out at the trans connection....

[JRANGER]

brand new cable shouldnt need lube but thanks