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Speedo cable could have come loose from the back of the instrument cluster or from the transmission. It could have also snapped.

I snapped one in a Celica. The insulation had been damaged allowing moisture to destroy the cable with rust.
 

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Speedometer= Thing that tells you your speed.

Tachometer= Thing that tells you how fast your engine is spinning.

I would also have to go with the speedo cable. First thing to check is if it fell off the back of the cluster (the thing that has all the gauges in it), then check to see if the cable is broken (both were already said), if those two check out, pull out the speedo driven gear (it is what the cable attaches to on the transmission), by removing the 10mm bolt and holdown piece and twist/pull it out and see if the teeth are stripped.
 

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i had a question about the speedo drive gear. are ther different ones available with a different tooth count or is their just the one?
 

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The gearing in the tranny is related to the ratio of the rear end. You cannot also just swap the driven gear from one tranny to the other unless if they belonged to the same ratio of diff. The way that Toyota designed the gears in the tranny to work with the ratio is the drive gear on the output shaft changes diameter with how low the ratio is, and a direct relationship of that is that the driven gear got smaller when the diameter of the drive gear gets larger. And the only way to swap out both the drive gear is by taking the rear part of the tranny off.
 

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is the speedometer needle bouncing around in 5 mpg increments a sign of anything or is this normal for our cars?
 

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is the speedometer needle bouncing around in 5 mpg increments a sign of anything or is this normal for our cars?
More than likely you are getting some rust in the tubing of the speedo cable. Take it out of the car and flush it out with a healthy does of WD40 and then get some white lithium spray grease and have at it until it comes out the other end (I have "fixed" a few this way).
 
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