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Discussion Starter #1
Hi there, I've swapped my Powerglide for a TH350 in my 327 powered Impala. As anticipated, the speedo now over reads showing 75mph at 60mph. I checked the drive and driven speedo gears and I have a 21 tooth driven gear, and as best I can tell what looks like a pink or white drive gear. Because of my exhaust set up it's not easy to see into the trans and count teeth etc. A pink looking gear 'should' be a 7 tooth.

Now I've checked out the gear calculators that take into account the diff ratio and tire diameter. According to the calculator I would need a 17 tooth driven gear if I keep the 7 tooth drive gear. And if I keep the 21 tooth driven gear I would need a 9 tooth drive gear. So that's all fine, I can get one or the other. I back calculated and worked out that I probably have a 7 tooth drive gear because that would cause about a 25% increase in speedo reading.

The problem is that generically most websites say that if your speedo over reads then you need more driven gear teeth to slow it down. Or alternatively less drive gear teeth. So increasing the driven gear reduces the shown speed by about 2mph per tooth. (so on that basis I'd need a 28 tooth gear) Or reducing teeth on the drive gear reduces 10mph per tooth. But you can't get less than a 7.

So now I'm confused. I can order a 17 tooth driven gear and see how close I get, but I'm in NZ so it's not all that straightforward to ship gears to try them out and then get more. I just can't work out why the calculator indicates I need less teeth when all the gear providers say I need more. I could swap both gears and according to the calculator it would be right, but then that would be at odds with general convention. Ideally I would just swap the driven gear cos it's easier to get to. Has anyone else had a similar situation? Maybe my tire/diff combo just means my requirement is for less driven teeth?
 

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What is your tire height and axle ratio?

And yes, you are correct, the more teeth you put on a driven gear, the less the speedometer will show, regardless of what some sites are telling you! The smaller the driven gear, the greater the speed showing.
 

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It's all math, except for taking the tail shaft off and physically swapping the gears. I would give these guys a call:


They will sell you the correct gear set for your final drive gear ratio in the rear end with tire size and you don't have to do the math.

Big Dave
 

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Discussion Starter #4
So I'm running 245/60/15 tires with a diameter of 26.6". This equates to 759 revs per mile. The diff ratio is 3.08. So using the calculator on the PATC site, the required driven gear is 16.3 or 17. That's the same result I got from a different site. I currently have a 21 tooth driven gear, so according to the calculator I need less teeth. That's the bit I don't get. On this basis my speedo should be under-reading, but I've checked the actual speed with a GPS and the speedo is reading 15mph over at 60 mph.
 

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I'm in the GM parts book under 1969 Chevrolet TH350 (not that the year matters).

It lists an F70-14 tire (26.24") and a G70-14 tire (26.82") with a 3.07 gear ratio (almost exactly your gear). With those tires and those gears, GM installed a 6261076 drive gear (9 teeth) and a 3987919 (19 tooth) driven gear from the factory.

Your tire height falls in between those 2 tires for height, so that should be fine.

Your gear ratio is almost identical, just a fraction of a percent off, so that should be fine.

I am convinced that you need a 9 tooth drive, 19 tooth driven gear.
 

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Discussion Starter #6
Thanks Darth and Big Dave. The calculator suggests a 9 tooth drive gear with the 21 I have so I think what I'll do is go with changing the drive gear first and see how that ends up. Then if the 21 isn't right I'll go with a 19 driven gear. Because my wheels aren't stock there's every chance my speedo wasn't reading correctly before I changed the trans, so it will be interesting to see how far off it was before.

My speedo needle also swings like a metronome so I think I should remove the cable and lubricate it while I'm doing all this. I'll report back once the gears are replaced and see how it worked out.:)
 
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