Speedometer Gear...

[Elliot949]

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

[davepl]

Good place to start:

http://www.tciauto.com/tc/speedometer-gear-calculator/

[Elliot949]

Originally Posted by davepl

Good place to start:

http://www.tciauto.com/tc/speedometer-gear-calculator/

Thanks Dave but I don't know the # of teeth

[davepl]

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....

[Elliot949]

Originally Posted by davepl

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:

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.

That is the problem I have nothing now... all my stuff was stolen so I am trying to get everything pieced together... Math isn't one of my strong points... LOL

[davepl]

Originally Posted by Elliot949

That is the problem I have nothing now... all my stuff was stolen so I am trying to get everything pieced together... Math isn't one of my strong points... LOL

Well you've got a transmission, right? You want a gear to work with it, you need to know what's in there or be able to replace it with a known quantity. It's much easier to look.

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.

[Elliot949]

Originally Posted by davepl

Well you've got a transmission, right? You want a gear to work with it, you need to know what's in there or be able to replace it with a known quantity. It's much easier to look.

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.

Thanks... I am sitting at home and the truck is at the shop so I will have to wait then... I was hoping to get everything ordered today so I don't hold up the shop and have anymore delays than what I am already dealing with... LOL... I really appreciate your help...

[davepl]

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).

[Elliot949]

Originally Posted by davepl

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).

I got it... Thanks... I will just have to see if I can find out...

[swamp rat]

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+