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#1 |
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Location: Winfield, Ks.
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Speedometer Gear...
Hey Guys... I need a little help here... I am trying to get the right speedometer gear for my truck...
I am running a SBC (327) with a Turbo 400 trans and 308 Gears... The rear tires are 295/50x15's... What Speedometer gear do I need for a accurate speedometer... Thanks... Dan
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Dan Johnston... Owner of My Dad's- '67 Chevy C-20 Custom Camper Short Fleetside Pickup PAPA J And our newest addition a- '71 Chevy CST/10 Short Fleetside Pickup Haulin' SOLD Papa J's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=612847 Haulin's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=671130SOLD B Bears Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=744210 |
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#2 |
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Join Date: Aug 2006
Location: Redmond, WA
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Re: Speedometer Gear...
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#3 | |
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Re: Speedometer Gear...
Quote:
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Dan Johnston... Owner of My Dad's- '67 Chevy C-20 Custom Camper Short Fleetside Pickup PAPA J And our newest addition a- '71 Chevy CST/10 Short Fleetside Pickup Haulin' SOLD Papa J's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=612847 Haulin's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=671130SOLD B Bears Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=744210 |
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#4 |
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Location: Redmond, WA
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Re: Speedometer Gear...
What you do is plug in all of your other information and it will crank out the number of teeth you need. Then you look on the chart to find out what color those are.
I think you have all the info you need there (except tire diameter you might have to calculate, measure, or look up). Should be 26.6" for a normal 295R50-15. You -could- likely just pick a drive gear and say "I'm going with 15 tooth" but it's easier if you look and see what you have now, then you can change only one. Otherwise I guess pick 15 and see if you can make that work mathematically. Most older GM speedos are 1000 revs per mile. So, using a simpler one at: http://www.gtsparkplugs.com/SpeedoGearCalc.html 15 driven teeth and 35 drive teeth. That's one brown stock gear and one pink 35-tooth. But check my math, that was a quick walk through it. Problem is you can't change the drive gear I don't think without blowing the trans apart, as it goes around the entire output shaft, so you'll really need to shine a light in there and find out what you've got now. Odds are it's 15 but I hate to guess. I've never done this myself, so hopefully someone more experienced will wander by this thread soon....
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#5 | |
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Re: Speedometer Gear...
Quote:
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Dan Johnston... Owner of My Dad's- '67 Chevy C-20 Custom Camper Short Fleetside Pickup PAPA J And our newest addition a- '71 Chevy CST/10 Short Fleetside Pickup Haulin' SOLD Papa J's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=612847 Haulin's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=671130SOLD B Bears Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=744210 |
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#6 | |
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Location: Redmond, WA
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Re: Speedometer Gear...
Quote:
Or do you not have a trans yet? Then you can't really do anything unless you can somehow spec the gear to use in the one you're getting.
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#7 | |
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Re: Speedometer Gear...
Quote:
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Dan Johnston... Owner of My Dad's- '67 Chevy C-20 Custom Camper Short Fleetside Pickup PAPA J And our newest addition a- '71 Chevy CST/10 Short Fleetside Pickup Haulin' SOLD Papa J's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=612847 Haulin's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=671130SOLD B Bears Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=744210 |
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#8 |
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Join Date: Aug 2006
Location: Redmond, WA
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Re: Speedometer Gear...
You should be able to pull the speedo cable, pop out the plastic driven gear, and shine a light in to see what color the plastic drive gear is. Problem is the plastic drive gear is captured and can't be (easily) changed which is why you have to find out what it is and work with it.
If it's new to you, there are two gears inside. One is the drive gear, wrapped around the output shaft. It has some number of teeth (like 15). Ther other is the driven gear, which you can easily change. That pair of gears makes up the ratio you need. Here in this pic you can see the drive gear in green and the driven gear in grey. That should make it more obvious why you need to know what the green one is before you can pick the grey one (colors change depending on the number of teeth).
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#9 | |
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Re: Speedometer Gear...
Quote:
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Dan Johnston... Owner of My Dad's- '67 Chevy C-20 Custom Camper Short Fleetside Pickup PAPA J And our newest addition a- '71 Chevy CST/10 Short Fleetside Pickup Haulin' SOLD Papa J's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=612847 Haulin's Build Thread: http://67-72chevytrucks.com/vboard/s...d.php?t=671130SOLD B Bears Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=744210 |
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#10 |
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Join Date: Oct 2007
Location: Spanaway
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Re: Speedometer Gear...
You could just buy an adapter like what is used on the K20's most calibration shops carry them, very similar to the E-Bay pic in this search.
https://www.google.com/?gws_rd=ssl#q...ear+reduction+
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