Fixing the speedo via a plastic gear?

[Blackdog3]

Hey gang, i am not sure if this belongs here but.... again i have the trans out of my truck 3rd TC not sure whats going on anymore besides bad spacing causing a failure but i got a question.....

A buddy of mine was telling me there are plastic gear that slide in or on the input shaft to tell the speedo what speed you going if you have larger tires on your rig, is that true? If so since its out again, might as well look into it.

Thanks

Chris

 

[oneluckypops]

if your truck is a 4X4 wich i thought it was, The speedometer gear will be in the tail housing on the rear of the Transfercase NOT the Transmission. Theya re a plastic radius cut beveled gear.

 

[HotRodPC]

And even if its 2wd, it will be on the output shaft, not the input shaft. Yes, its true, you can change the drive gear and the driven gear to get a differant ratio to calibrate your speedo. If I knew your tire size, and gear ratio, I can likely figure out which gear colors you need. Also specify which trans or transfer case.

 

[GreaseDog]

And even if its 2wd, it will be on the output shaft, not the input shaft. Yes, its true, you can change the drive gear and the driven gear to get a differant ratio to calibrate your speedo. If I knew your tire size, and gear ratio, I can likely figure out which gear colors you need. Also specify which trans or transfer case.

same here, i've got GM catalogs with all of the gear part numbers/colors in them. if i knew what drive gear, driven gear, factory gear ratio, current gear ratio, and current/desired tire size, i can show you exactly which gear you would need.

 

[Blackdog3]

Ah see that shows how much i know haha. I have 4.11 gears and a NP205 t case on 35's

Thanks

Chris

 

[HotRodPC]

You need a 15 tooth drive gear, and a 35 tooth driven gear. Chances are you have the 15 tooth drive gear on the output shaft of the Tcase already, so now you'll just need to pull the speedo driven gear out of the side of the Tcase and install the 35tooth driven gear which should be the orange colored gear.

 

[Blackdog3]

Alright cool, so its in the t case not the trans? Where do i get such things?

 

[c1084]

Hi, new member here. I have a similar question, already posted it in the introduction section. I'm hoping you might be able to help? I have an 84 C10 with a 700R4 and 235/75/15 tires, rear axle is 2.73. My speedo is reading 18-20 mph too fast. I suspect someone changed the trans and the donor vehicle had larger tires or different axle ratio. How do I tell which gear should be in my truck, and where's the best place to find it? Any info greatly appreciated!

 

[HotRodPC]

Hi, new member here. I have a similar question, already posted it in the introduction section. I'm hoping you might be able to help? I have an 84 C10 with a 700R4 and 235/75/15 tires, rear axle is 2.73. My speedo is reading 18-20 mph too fast. I suspect someone changed the trans and the donor vehicle had larger tires or different axle ratio. How do I tell which gear should be in my truck, and where's the best place to find it? Any info greatly appreciated!

Doing some nasty ass math with the formula when and if I can find it is how you figure that out. Whatever you do, just don't fall for the "take the needle off and scoot it back" Change the gear or possibly gears. There is 2 different driven gears, and several different drive gears. I'll see if I can dig up the info. If I haven't gotten back in a day or so, bump the thread again.

BTW, Welcome to GMSB.

 

[SlickGTP]

Hi, new member here. I have a similar question, already posted it in the introduction section. I'm hoping you might be able to help? I have an 84 C10 with a 700R4 and 235/75/15 tires, rear axle is 2.73. My speedo is reading 18-20 mph too fast. I suspect someone changed the trans and the donor vehicle had larger tires or different axle ratio. How do I tell which gear should be in my truck, and where's the best place to find it? Any info greatly appreciated!

I posted some links on your intro thread...

If I can get the pics up, I'll do a write up on how I changed mine. It was a TH400, but should be a similar process regardless.

 

[HotRodPC]

Here's the formula to figure it out.

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Do you know what color gear you have in the side of your trans case? That will tell us the # of teeth on your drive gear so we can calculate what driven gear you need. You tire size is a 28.88 in tire, so you calculate that at 29 since radials grow a bit anyway.

You must be registered for see images

 

[c1084]

And even if its 2wd, it will be on the output shaft, not the input shaft. Yes, its true, you can change the drive gear and the driven gear to get a differant ratio to calibrate your speedo. If I knew your tire size, and gear ratio, I can likely figure out which gear colors you need. Also specify which trans or transfer case.

Then perhaps you could help me? 235/75/15 tires and 2.73 rear diff gears in an '84 c10 2wd w/700c4?

 

[HotRodPC]

Then perhaps you could help me? 235/75/15 tires and 2.73 rear diff gears in an '84 c10 2wd w/700c4?

Sure, Let me know how many teeth you have on your Drive gear. That's the gear that the speedometer is plugged into. You'll need a wrench to pull the speedo drive gear housing out of the trans tailshaft. Using the chart above, the color should be a big clue as to what tooth count is. But do count the teeth to be sure. Sometimes they fade in color from the heat and being used, and they can look a different color than they really are. Usually on the lighter colors is when this will happen.

 

[c1084]

Will do sometime in the next few days. I'll just be getting the truck back tomorrow after replacing the engine, complete exhaust system and complete brake overhaul. Thanks again.