Driving me Crazy...and over the speed limit.

Speedo gear is wrong. Bouncing around is typical and it sucks. Lube up your cable, if that doesn't solve it replace it.

My speedometer bounces too, but that's cause it only goes to 85, and I've gone alot faster than that!!!

The speedo in my '86 has started to bounce lately when the temps get below 45*F. Probably just the cable, as it's the original 21-year-old piece.

did the guy put a gear in when he had the 4.10's put in?

foxmustangman said:

1.235/45-17
2. 21 teeth-red gear
3. trans gear is black

Oh, and I found out that I have a 4.10 rear end.

Transmission is AOD.

Thanks.

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Your tires have a diameter of 25.33 inches (Tire height = Tread width x Aspect ratio / 1270 + Wheel diameter), which is much smaller than stock.

A 1991 AOD should have an 8-tooth speedometer drive gear that is not interchangeable because it is machined onto the output shaft. If you have a 21-tooth driven gear in your AOD, it should be purple. A red, 21-tooth gear is for a 5-speed tranny (Could that explain the bouncing? Anyone?)

With a 4.10 rear end, you would require a 26-tooth speedometer gear to make your speedometer accurate (# Driven gear teeth = 20.168114 / Tire height x Axle ratio x # Drive gear teeth). Unfortunately, a 23-tooth driven gear (white) is the highest one you can get.

Right now you are looking at a nearly 20% error in your speedometer readings. Changing to a 23-tooth gear will reduce that error to about 12%, but will not get it completely accurate.

Buy several 23-tooth gears. They generally do not last very long. The teeth are very thin and break easily, causing a bouncing speedometer needle.

TRWXXA said:

A red, 21-tooth gear is for a 5-speed tranny (Could that explain the bouncing? Anyone?)

With a 4.10 rear end, you would require a 26-tooth speedometer gear to make your speedometer accurate (# Driven gear teeth = 20.168114 / Tire height x Axle ratio x # Drive gear teeth). Unfortunately, a 23-tooth driven gear (white) is the highest one you can get.

Right now you are looking at a nearly 20% error in your speedometer readings. Changing to a 23-tooth gear will reduce that error to about 12%, but will not get it completely accurate.

Buy several 23-tooth gears. They generally do not last very long. The teeth are very thin and break easily, causing a bouncing speedometer needle.

Click to expand...

all the driven gears are interchangeable between 5spd and automatic, as is the speedo cable. the bouncing speedo is, as mentioned above, usually a cable problem, a little lube or replacement should do the trick.

also, with the 4.10's go with the white gear, i used that combo with the stock aod in my car and it worked fine. actually get an frpp one though, i got one off of summit (tci i think) and there was so much casting flash on the inside that it didnt want to go on the cable. a little exacto knife work solved it, but still a pita.

Chuckman said:

all the driven gears are interchangeable between 5spd and automatic, as is the speedo cable.

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Are you sure about that?

View attachment 338633
5-speed speedo gear.




AOD speedo gears.

foxmustangman said:

Okay. Cool, thanks guys. The guy who put in the 4.10 got the gear as close as he could...which apparently isn't close enough. And I can tell how fast I'm going by looking @ my tach, so we're good.

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If you get any transmission work done in the future, for about $50 you can get a reconditioned, pre-1989 AOD output shaft with a 7-tooth drive gear on it, and get it installed. That and a 23-tooth driven gear will get your speedo error down to less than 2%.

foxmustangman said:

...but if I can save $20 by just lubricating it, then there's money well spent. What do I use and where do I put it? (no sexual pun intended)

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I had to be the first to laugh

4.10's and and AOD....there is no speedo gear to correct it perfectly.

Throw a white 23 tooth speedo gear in there and that is the cloesest you will get.

As far as lubing the cable....squirt some oil down inside there from the speedo area, and let it drip down the bottom.

Get a tube of graphite speedo cable lube from the local auto parts store.
Put newspaper or a paint dropcloth all over the front part of the interior of you car. Make sure you have plenty of paper
towels to clean yourself up with. This can be a messy job.

Remove the speedo cluster and remove the cable from the back of the speedo head by depressing the white clip on the
speedo cable.

Speedo head cable clip

Photo courtesy of Almost Stock




The cable and head will separate. Pull on the center part of the cable and is will slide out. Pull it all the way out because you
need to get to the transmission end of the cable.

Now you have the nasty, dirty cable in you lap and all over the carpet. Aren't you glad you took my advice about covering
the inside of the car with a dropcloth or newspapers? :

Coat the lower half of the cable with the graphite lube. Feed the cable back into the housing. This will scape the
excess lube off and help lube the top part of the housing. The idea is to put most of the lube on the bottom end of the
cable because that is the part exposed to weather. If you get too much lube on the top end, it will work its way into the
speedo head and get between the rotating magnet and the drum that is connected to the pointer. When that happens, the
pointer may peg out against the stop or break something.