I have an error in my speedo which I want to correct. The driveline is a Chev v8, Powerglide, 2.78 ratio Salisbury and 215/65 14 tyres. The gear on the speedo cable is brown, but I haven't counted the teeth. The GMH shop manual (Monaro supplement) for HK series and my combo lists Drive gear as part 3915003, Natural colour and Driven gear as part 3860341, brown colour with 18 teeth. So this appears to match what I have but results in a speedo error. However, the HT supplement for my combo lists Drive gear as 3915004 Blue colour and Driven gear as 2808567 colour white which appears to be different gears. Which listing is correct and why are they different? Thanks

Thanks. It's very possible I have an HK 'glide because it's not the original transmission, however I have the correct HK gears. Perhaps they aren't right for my speedo? The other possibility is that I have an HT box,but an HK driven gear. I'll check the box date of manufacture and drive gear (if possible) which should clarify things. Thanks again.

Yes, the speedo error is around 10% (as I recall but I haven't driven the beast for many years). Working on the theory that the error is caused by having an HT box and an HK driven gear, then I should be able to fix it by using the HT gear listed for my combo. This gear is listed as 2808567 which appears to have 18 teeth which is the same as the brown one currently in use. So I'm not expecting that changing to this gear will fix the problem......or am I missing something? Haven't been able to check the gear in the box cause the car is away.

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Originally posted by gm5735
Not so fast!
There are several different drive gears used on Powerglides, and on other Chev sourced transmissions including Saginaw and Muncie. Given we don't know where the glide came from, or its history there are a few possibilities.
Just to make it difficult, the two main drive gear variants are both 8 tooth, but of different outside diameters.
The two main ones for V8 glides are:

3915003 = 3708144, which is 8 teeth, 1.84" diameter,
and
3915004 = 3708145, which is also 8 teeth, but 1.76" diameter.

The 1.84" drive gear needs to be used with a driven gear of 0.8" diameter,
and the 1.76" drive gear needs to be used with a driven gear of 0.95" diameter.

A mismatch might even work for a while, but usually kills one or both gears, leaving lovely plastic chips in the transmission and a non functional speedo.

Which brings me to the HK Shop manuals. Some of the recommendations in there seem to be incorrect, matching incorrect gears. This may be the real reason for the HT using only the 3915004/3708145 drive gear. That, and the fact that there is a 17 tooth gear available for the 3915004/3708145 combination whereas there is not for the 3915003/3708144.
Unfortunately for you, the 18 tooth driven gear for both drive gears is brown, so colour alone will not identify it. When you pull it out you will need to measure its outside diameter which, assuming it isn't damaged, will tell you what gear is in the glide. (Unless of course you want to pull the glide apart and measure the drive gear OD.)

Once you know the diameter of the driven gear, which you can measure, and advise in which direction you need to correct the speedo, I'll give you my best guess for the gear you need.
If you really do have a 3860341 driven gear and matching drive gear, and a speedo which reads low you may have a problem, as I don't think there is a matching 17 tooth driven gear to suit, meaning you may need to change the drive gear in the box as well as the driven gear. (Or the diff ratio!)

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And that Ladies & Gentlemen is what is called "knowing your sh!t".

It's nice to hear from someone who knows their subject.

Very good info there, gm5735, thank you very much.

Dr Terry

Thanks Dr Terry, but I assure it was a rare moment of lucidity, and unlikely to be repeated.

Skidmark, is it possible that the "brown" you refer to is actually faded red? This would make the driven gear a 3987921, to suit a 3915003/3708144 drive gear. The part number may or may not be on the gear. The 0.8" measurement confirms that this is the smaller diameter driven gear to suit the larger drive gear.

We can go two ways for the next step, getting you the right gear.

Option 1.

215/65 14 tyres will do 806.6 revolutions per mile. Trust me, I'm not a salesman, and it will.
Therefore, to travel 60 miles the rear wheels will turn 60 x 806.6= 48,396 revolutions.
In that 60 miles your tailshaft, with a 2.78 pinion set will turn 48,396 x 2.78 = 134,540.88 times.

This equates to 134,540.88/60 = 2242.34 RPM, which will also be your engine speed at 60mph.

GM Mechanical speedos are standardised to work with 1001 revolutions per mile.
This also means you will have 1001 RPM at the speedo at 60mph.
The correct speedo gear set will need to be in the ratio of 2242.34/1001 = 2.2401
Since we have an 8 tooth drive gear, the driven gear will need to be 8 x 2.2401 = 17.92

Which means use the 18 tooth gear, and have a 0.5% speedo error.
(Half a percent is fine. Standard GM tolerance used to be -0%, +5%, meaning the speedo reading up to 5% high is OK)

In an amazing puff of GM logic, the last two numbers of the part number equate to the number of teeth on the gear,
therefore the part number, in 0.8 inch, is 3987918

Option 2 is much easier, and involves using the charts in the manual!
They confirm 18 teeth, but in my opinion, and confirmed by your measurement, you need a 0.8" diameter gear and not the 3860341 specified in the HK manual. The 3860341 is 0.95" diameter. Also, don't be tempted to use 2808567, which is the 18 tooth specified for HT and other later gearboxes, and which is for the smaller drive gear. I've never checked, but I think it will also be 0.95" diameter.

Out of interest, your 21 tooth gear would indicate 51.2 mph at a road speed of 60mph.

The bad news is that you would have received many speeding tickets.
The good news is that the 25,000km that you think you have travelled, from your odometer, is really 29,296km, and also changing the speedo gear will make you think you have increased your fuel economy by 15%.

I hope that helps.

Geoff

Edited by user Friday, 25 July 2014 11:15:24 AM(UTC)  | Reason: Not specified

No worries. Must've been a full moon. At the risk of teaching you to suck eggs, please do make sure what you end up buying has a long enough shaft to work with your speedo cable ie physically the same as the one you took out.
As we don't know the history of that glide, there are some discrepancies, and that I've heard stories about problems with gears for the very early glides just be a little cautious.
Good luck.