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Recaroguy,

I recently had a similar problem with my son's '82 camaro. However our problem was that the speedo was 15-20 mph higher than the actual speed. In our case, It turns out that a previous owner had the ring/pinion combination changed to a 3.73 (11:41). He had also changed the tranny from a 1982 TH-200, to a 1984 TH-700R4. He never took the time to correct the transmission's Speedo Drive Gears to compensate for his changes

It is extremely unlikely anything is wrong with the Speedometer. It seems very likely that the rear gearing was changed in your car. Someone will certainly correct me if I'm wrong but I ASSUME the gearing went DOWN. The only way to verify 100% the gearing is to remove the rear axle cover and find the gear stamping on the ring & pinion. You should also do a physical count of the teeth on each. The ring and pinion will each have a part# (if they're GM) and a tooth count (i.e. 11:41) stamped on them. Once you know the actual gearing you can then get the proper drive gears for the speedo.

Inside the tail section of the tranny (TH-200?) there is a plastic gear on the actual tailshaft and a gear inside the Speedo Housing that is driven by it. There are dozens of possible tooth combinations and gears. Each is a different color (see photo) and you'll need to select the proper gears to adjust for your rear's gearing. We needed a 15 tooth / 45 Tooth combination to get my son's properly calibrated for the 3.73 rear end.

Once you count/verify the rear end your local Chevy parts guy (or someone here) can tell you what speedo drive gears you will need. Each gear is $6.24 (x2) from the GM dealer. The only other possible problem you could run into is that some of the Speedo housings have maximum gear count limits. In photo #2 you will see that the Aluminum housing is stamped 40, 41, 42, 43. I screwed up and "Stuffed" a 45 into the plastic housing and it snapped off in less than five miles. You may have to hunt the local J.Yard for the proper housing as a new housing is $66 +/- from GM. We found one for my son's at the J.Yard for $5.

To the best of my knowledge NOTHING inside the tranny can effect the speedo.... UNLESS the guy that rebuilt it didn't transplant your plastic gears. Often, tranny shops install a transmission off the shelf they have ready for install rather than rebuilding your tranny.

Sincerely,
Kurt

 

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It's not true at all. There's no direct connection between the speedometer cable and the needle that could cause it to break.

Regardless of whether you had 2.73s or 2.93s, any "wrong" rear end gears they put in would be lower ratio than what you had originally. This would make your speedometer read higher than your actual speed. If the speedometer gears in the transmission are correct, then the problem is with the speedometer head itself.

 

judging by how far off your speedo is off the either rear-end shop probably put in a completly diffrent rear end gearset, (which is inprobable, since U said it origanally had 2.73 or 2.93 which are close enough not to through off the speedo too much if U get the wrong one), or when your speedo gears were replaced they used the wrong gears, also the speedo stopping needle WILL NOT cause the plastic gears to break, the place you had change them probably diden't properly change out both gears, i.e, put a new driven gear (the one that atteches to the cable) but diden't take the time or effort to remove the tailshaft and change the drive gear (the one that goes in the output shaft), which will accererate were on the new gear, they also probably diden't use the proper driven gear which would also throw off the speedo, they probobly just got any random driven gear off the shelf, insted of matching it to the rear end ratio, and an 82 had 5 diffrent ratios avaiable, they are... 2.73, 2.93, 3.08, 3.23 and 3.42, and they could have used a speedo drive set from any of those ratios. also U might want to varify your factory ratio, it could have been any of the above ratios, the axle code (I don't have them any more....but I'm sure someone on the bords has them) is on the forward portion of one of the axle tubes (I beleve it is the drivers side one...but I'm not 100% sure) and will tell you what the car came whith stock, and you could compair it to the ratio that was put in.

 

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recaroguy,

Even if you were to find the original numbers stamped into the axle (on the front side of the pass side) which are probably covered with 1/4" of road gunk and possibly undercoat, there is no guarantee that what it says on the outside, is what is actually INSIDE!

In my personal opinion, the first step is rather easy and costs you almost nothing. You should have a 10 Bolt rear end. That means you just have to remove ten little bolts and LOOK inside to see what you've got. It's not that big of a job. I've found that neither AutoZone nor Advance had a new gasket in-stock so you may have to make a new one....also no big deal. Replace the gasket put the ten bolts back in and fill 'er up with 80# gear oil.

Once you have determined FOR SURE what is in the back end. You can post that and we can figure out what the tranny gears should be......

Checking what the plastic gears are is pretty simple also. There is only ONE(1) 10mm bolt holding in the speedo housing. Remove it and the retaining clip. With a small amount of effort, the housing will just pop out. You can easily count the teeth on the drive gear and the axle gear by looking through the hole.

You might be best served by letting a good shop change the shaft gear if your not comfortable with that one. It will be a little more complicated and maybe you would be better off spending the few bucks.

Just so you are aware, a friend of mine owns/operates a transmission shop in PA and it's one of the Big Name National Franchise Places, a name everyone has heard on TV. They almost NEVER repair YOUR tranny..... If you have a TH 300, 350, 400, 700 they've got ten of them ready on the shelf and you get the first one they grab after they remove yours. "YOUR" tranny ends up in some other customer's ride a couple weeks later. Hopefully, the technicians are smart enough to verify the speedo gears?

Sincerely,
Kurt

 

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you still need to take the tailshaft off to change the drive gear, kboehringer was refering to taking out the driven gear, if the drive gear is mis-mached to the driven gear or it is badly worn it will just chew up the driven gear

 

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recaroguy,
Even if you have to ask a "Professional" to make the repair for you, now you know what "should be" happening under the car to correct the problem properly.

As 84 Z-28 350 mentioned, I also strongly suspect that the "DRIVE GEAR" may not be in good condition and that COULD be causing the continuous failures to the speedo. The other large contributing factor COULD be a housing that is not appropiate for the DRIVEN gear being installed in it. Most of the housings have numbers cast into them indicating the size limits, THAT MUST BE VERIFIED!

[IMPO] A bit of additonal advice.... DO NOT LEAVE THE CAR AT THE REPAIR FACILITY! Stay and watch closely if you are permitted, peek through the garage door windows if you must! In order to change the DRIVE GEAR, the Drive Shaft, Torque Arm, and Tail Housing MUST be removed. If you don't see the drive shaft & torque arm removed.... You're NOT getting a proper repair!!!

Also, you can't spend too much time driving around without a speedo. The ECM needs the VSS signal. You're check engine light will illuminate the moment the speedo stops working.

Sincerely,
Kurt

 

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