Speedometer doesn't work when you let of the gas

[5.0zone]

OK, this is really strange. I have a 89 Festiva with a 4 speed. I just replaced the trans with another 4 speed. the speedometer works ok as long as you are accelerating or keep a load on the engine. As soon as you take your foot off the gas it drops to zero. Put your foot back on the gas and it jumps back up to the speed you are going. I replaced the part on the end of the speedo cable with the gear going into the trans. But it still does the same thing.
Any ideas? God I don't want to pull that tranny out again.

[mechanicaldj]

you may want to jack up the front of the car, put it on jack stands

have a friend start and drive the car
and verify your problem still exists

if it does then:
take the cable of of the speedometer gear housing
drive the car in the air again
and watch the center the speedo housing
if it turns all of the time the wheels are moving, including deceleration
it is your speedometer head itself
but if it stops turning on deceleration, you have internal gear or bearing issues inside your trans

[5.0zone]

Ok, so the problem ended up being something in the transmission.
Replaced the transmission and now it works fine.
One day I may pull it apart just to understand why.

But I got a great deal on the second tranny, free.
Just had to tow the car out of my mother-n-law's yard.

[packrat427]

Almost sounds like a c-clip came off letting the speedo gear in the trans move around and dis-engage.

[mechanicaldj]

differential bearings gone bad