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

[foxmustangman]

Okay, so i realised that i'd gotten so used to my spedo. that i forgot to ask what the heck the problem with it is.

My spedo is off. when it says 25...i'm going 20
It says 66, i'm going 55,
it says 107 and i'm really only going 91 mph.

Do i just need a new spedo gear?

Plus when it's cold, it just bounces all over the place. And when my trans finally gets warm it'll stop.

 

[fivespeedsteed]

i souns like a gear then, the tire size or rear gears could have thrown this off. as for the bouncing, i dont know, most ones that i see bounce are just becasue they are so horribly off, they keep maxing out, back to zero, ect..

 

[Mustang5L5]

Bouncing speedo's tend to be the cables drying up and friction causing them to bind up.


As for why your speedo is off, well it could be any number of things.

what size rear tires are you running, what is your trans? what rear end gears do you have? What speedo drive gear is currently inside your transmission (will require crawling under to check)

 

[foxmustangman]

Yeah, I got 17 inch wheels, but the speedo is the same as compared with my 16 inch ponies. As for the trans., the gears, and spedo drive...i have no idea. As far as i know they should all be stock, unless the guy before me changed something and didnt tell us when we bought it.

 

[Mustang5L5]

Well, if you want to fix this issue, time to do some digging.

#1) Tire size
#2) pull speedo cable out of trans and tell us the color and tooth count (color is really all you need as i have a chart)
#3) look inside the hole the cable came out of on the trans and tell us what color gear is inside there (yellow, green or black)

With those 3 peices of info, i can tell you what is right or wrong with the setup.

 

[SVT32VDOHC]

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

 

[81elky350]

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

 

[foxmustangman]

#1) Tire size
#2) pull speedo cable out of trans and tell us the color and tooth count (color is really all you need as i have a chart)
#3) look inside the hole the cable came out of on the trans and tell us what color gear is inside there (yellow, green or black)

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.

 

[Darkwriter77]

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.

 

[ryan218]

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

 

[TRWXXA]

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.

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.

 

[Chuckman]

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.

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.

 

[foxmustangman]

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.

 

[TRWXXA]

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

Are you sure about that?

View attachment 338633
5-speed speedo gear.

AOD speedo gears.

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.

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]

Actually, the red one is the exact same as the one I pulled out of my trans. So they're probably interchangeable.

 

[foxmustangman]

Okay, bringing this one back up since I have yet to actually fix this. I believe since this thread was last posted in I got my new (err...replacement) transmission from an 89 Mustang. I aslo now have 27" tires. So the 23 tooth gear should work fine.

Now, how do you lube up the cable? I'm thinking about just getting a new one, 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)

 

[fiveohwblow]

...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)

I had to be the first to laugh

 

[SVT32VDOHC]

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.

 

[foxmustangman]

Thats what I'm gonna do. I'm just wondering now how to lube the cable.

 

[jrichker]

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.