It’s a possibility that the whole original rear end was swapped by a PO rather than just gears replaced. I’m willing to bet that your 4 speed is original to your 64, which means 66 and later Speedo pinion gears will not fit as suggested by several members. Here is what the business end with the 65-earlier pinion gear looks like:
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Speedometer pinion gear sizes were based on tire size and rear end gears. Here is the tire size/pinion gear chart for 65; for the A Body the only tire sizes available were either 6.50x13 or 7.00x13. The 14 and 15inch tires were only for the B Bodies and full size boats. You can probably use the smaller 14 inch tire size to figure the speedo pinion size you need if you’re running 14 in tires. Notice that with the 833, the 2.76 geared rear end was not available!
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They are not available new so used will be it. After you figure out which one you need, you might try Brewers. Otherwise post a Want Ad for the specific size you need. Hope this helps
