Updated Rear & speedo gears, now incorrect speedo

Ozz · May 20, 2004

I put in new rear gears, and waited to do my speedo correction gear. I put 3.73s in my AOD, and put an orange (20 tooth? or 21?) gear in my tranny to match it, based on what everyone told me to do.

In the meantime between the rear gears and speedo gear, I changed out my dash gauges for white facers. I adhered to everyone's warnings to let the speedo needle unwind by taking out the stopper peg at the bottom, then put it in the same place, wound it up, and put the peg back in the white gauge setup.

Now that I finally put the speedo correction gear in the tranny, my gauge reads about 10mph over what I am actually doing.

So if I DID do the gauge needle correctly, that leaves the speedo gear itself as the wrong one. I did what everyone said to, by using the orange one, but what other color should I try? more teeth or less?

Michael Yount · May 20, 2004

You'll need more teeth to slow the speedo down. Your only choice is a 23 tooth unit that is difficult to find. It should make the correction - however, many people report that the 23 tooth unit doesn't last very long. The alternative is to replace the drive gear inside the tranny along with the driven gear on the cable to get the right speed.

How did you measure your speedo error?

Ozz · May 20, 2004

I figured it while driving with my dad alongside me, going 70 while I did 80

Think it could be the needle, though? Or is the way I did it fool-proof?

91lxvert · May 20, 2004

http://www.geocities.com/mgnedell/notchback/tech/speedo.html

in 1990 they changed from a 7 toth drive gear to an 8 tooth drive gear in the transmissions.

the closest you can get to being correct is with a 23 tooth white speedo gear, you can order them from steeda

my speedo is about 6 mph off with 3:73's and the 23 tooth running 245/45/17s

the only way to get almost exact is to change the drive gear in the transmission to a 7 tooth which reqires disassembly.

TRWXXA · May 20, 2004

You need a 23 tooth gear (Late Model Restoration Supply has them too). I have know idea why folks would tell you that you need a 21 tooth gear, unless the didn't pay any attention to your signature.

The 23 tooth gear is the closest you're going to get (if you have anywhere near stock sized tires). If you do have stock tires, your speedo will read about 3% or 4% high. To get the speedo dead on, you'd have to be running 26 1/4" diameter tires.

Michael Yount · May 21, 2004

Also, I'd check the speedo error by mile markers on the Interstate and a clock, unless he's calibrated his speedo. There's no guarantee your dad's speedo is accurate.

Ozz · May 21, 2004

Thanks. Even if his speedo was off, it's still a heck of a lot closer than mine, haha.

I looked in my catalogs (before purchasing, too) and they all say that an AOD getting 3.73s should have an orange gear. I believe that before I changed that gear, I was 15mph over "actual" speed, so this barely helped me.

I'll pick up a 23-toother and hope that's the solution. Thanks!

TRWXXA · May 21, 2004

Michael Yount said:
Also, I'd check the speedo error by mile markers on the Interstate and a clock, unless he's calibrated his speedo. There's no guarantee your dad's speedo is accurate.

To simplify this method, you really don't need a clock. The odometer is driven by the speedo cable, it'll have the same amount of error as the speedo so you can just compare it to the mile posts. Use a good distance (5 or 10 mile posts) for best accuracy.

This way, you won't have to maintain an accurate and steady speed (nearly impossible to do on the interstates these days).

maverick0716 · May 21, 2004

Ford doesn't make the 23 tooth speedo gear but believe it or not Jeep makes one that will fit. Part # - j3167393

You'll never get the speedo bang on with the 8 tooth tranny and 3.73s but it will be fairly close.

leakyfaucet · May 21, 2004

I bought the white gear (forget if it's 21 spline or 23) for my car about 6 months ago. Recently, it started chirping at the dash really bad. I pulled it out and it had totally tore itself to shreds, but the speedometer was still showing correct speed (although the needle shook a little bit).

This was a steeda part BTW. Any suggestions?

TRWXXA · May 21, 2004

leakyfaucet said:
I bought the white gear (forget if it's 21 spline or 23) for my car about 6 months ago. Recently, it started chirping at the dash really bad. I pulled it out and it had totally tore itself to shreds, but the speedometer was still showing correct speed (although the needle shook a little bit).

This was a steeda part BTW. Any suggestions?

The teeth on the 23-tooth gears are very fragile. It is not uncommon for them to break over time (I've heard some people replacing them every year).

The white 23-tooth gear that came with my car was in pretty bad shape when I took it out (the speedo needle vibrated at low speed, but it still worked). I got a replacement from LRS. It is black in color, and it looks alot more like the stock speedo gear for the AOD. The one that I replaced looked more like the gear for a 5-speed tranny. We'll see how the new one holds up.

Ozz · May 21, 2004

I checked Steeda and they only offer (that I was able to find) was the T5 gear. I need the AOD gear w/the longer shaft. Is that the one Jeep makes? I can handle changing it out every year, it's easy enough and will just be another thing on the ol' maintenance list.

Michael Yount · May 21, 2004

As TRW alludes to, the high-tooth count driven gears are fragile - both 21 and 23 teeth. The ultimate solution is to change the drive gear on the output shaft - so you can use one of the 17-18-19 tooth gears on the cable. Not sure what's involved with the AOD - but with the T-5 in mine, the exhaust system has to come out of the way, driveshaft out, disconnect the clutch cable, remove shifter and drop the tranny. Then the drift pin that holds the shifter shaft must be removed, and the rear housing removed. Then it's a simple matter of removing a retaining clip and sliding off the old/on the new gear. Installation is the reverse. A really good mechanic familiar these cars/engines/trannys with lift and power tools, can probably do the whole thing in a bit more than an hour. But the 'book' probably pays 2 or 3 hours for a job like that. Anybody know what's involved with the AOD?

powershift351 · May 21, 2004

yeah, I had all kinds of problems with mine. I went with the speedware 200mph guage and I have 3.73. Big headache, but I am 2-3mph off on the 200mph speedo now. I can be done man

Ozz · May 21, 2004

I checked my Mustangs Unlimited catalog (they're local to me) and they only offer a 21 tooth gear for AODs. Is there a 23 tooth gear for AODs or just 5speeds?

jrichker · May 21, 2004

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire and rear end gear ratio you want. This will allow you to get the accuracy with within 2%-3% or better. The drawback is that it isn't cheap.

TRWXXA · May 21, 2004

mansonozz said:
I checked my Mustangs Unlimited catalog (they're local to me) and they only offer a 21 tooth gear for AODs. Is there a 23 tooth gear for AODs or just 5speeds?

Try the 23-tooth AOD gear from Latemodel Restoration Supply (catalog# LRS-17271J).

Their catalog is wrong regarding the required number of teeth for the 1990 and later AODs (they say the 23-tooth gear is for 3.55s), but the gear appears to be the correct one for the AOD. Since the first gear I had in my tranny was for a 5-speed, the teeth were broken at the ends nearest the cable. Since this would be more in the middle of the proper AOD gear teeth, hopefully they won't break off like that again.

The LRS 23-tooth gear is about twice the price of a stock gear ($10.95), but if it lasts longer I'm glad to pay it.

maverick0716 · May 21, 2004

This is the site I use to find out which speedo gears I need, and it's helped me out a lot.

http://www.flemworld.com/Hto/rear.html

Updated Rear & speedo gears, now incorrect speedo

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