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By Johnny B - 9 Years Ago
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I have a 56 merc Montclair with a 312 and mercomatic. The new speedometer cable is reading exactly 10 more mph than the speed I'm really going. Which size gear do I need for the trans? How many teeth? TY all for your input.
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By miker - 9 Years Ago
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If you go to the old manuals, it will give the gear theeth for each combo. The problem with old cars is many times the rear gears were changed, the tire height has been changed, even the trans got replaced and the gear not changed. Also many just weren't that accurate, the 55-56 birds were notorious for that.
We need to know where your 10mph fast-- 60mph?. Then you pull the existing gear, count the teeth, and figure the difference. If you've got 20 teeth and you're 10 over at 60, you need to slow it down 16%. 16% of 20 is 3.3, so a 23 or 24 will get you close. If they're available. Chances are you can get close enough to be useful.
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By speedpro56 - 9 Years Ago
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Sounds like you have a 19 tooth gear which was common back then. As mike said move up a few numbers in teeth count which should slow the speedometer down to the appropriate mph. If a 21 tooth then move up a few more numbers. Be sure to check your original gear before making a move.
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By Johnny B - 9 Years Ago
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Thanks for the help everyone. Just to let you know the 10 mph over is at all speeds.(needle solid and does not bounce) Will order a couple of new gears with more teeth.
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By miker - 9 Years Ago
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If you're 10 over at ALL speed (like at 10mph it reads 20?) it's not a gear problem. A gear problem is 5 over at 30 and 10 over at 60. Sounds to me like you've got a speedo problem, like maybe the needle was indexed wrong. Or the gauge face is off.
As an aside, the bird, speedo was the same as the passenger car, only the numbers went to 150 instead of 120. No wonder the birds where so fast!
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By speedpro56 - 9 Years Ago
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Did a gear change on my Tbird because main concern was my 65 mph accuracy and now I know my speed on the Interstate.
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