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T5 speedo gears, wtf?

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Old 04-10-2006, 08:20 AM
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T5 speedo gears, wtf?

I was up til 2AM reading posts on here. I still can't find anyone who has run a T5 with 3.73 gears and found the right combination for the speedo.

Some part #'s I see were posted for a 700R4, but those don't work in a T5..

So who has a T5 + 3.73 gears, and what drive/driven gear combo did you use to make your speedo work?

-- Joe
Old 04-10-2006, 09:27 AM
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I'm running 7 and 20 IIRC. Just the stock combo for my car.

Which incidentally, I moved over to my T-56 without incident.

I fail to see how it is possible that anyone with enough intelligence to count their fingers and toes at the same time, would have any difficulty whatsoever figuring this out. It's got to be one of the easiest things about a car to calculate. Easier by far than, say, compression ratio (not that most people who are pretending to calculate that, ever get it right; but that's a different rant..) If I, a total imbecile and moron, can manage it, then what does that say about people who can't?

Recall that your car has an odometer. This is the simplest of all devices. Think about it: all it is, is a turns counter. It counts how many times the cable turns. And, it's hooked up to THE SAME SOURCE OF MOTION as the speedo. Use this to your advantage.

The odometer has a ratio of 1000:1. Which means, 1000 turns of the cable, makes it advance by one mile. Pretty simple, eh? That's how a speedo service can calibrate your speedo when it's out of the car, WITHOUT YOUR SPEEDO GEARS; they know how fast to spin the cable to make it read whatever. 1000 RPM = 60 MPH. How how much more simple and mindless can it get?

So all you have to do is figure out how many thousands of times your driveshaft rotates in one mile, and get a set of speedo gears in that same ratio. That's all there is to it.

Go measure your wheel radius; NOT "calculate" the tire diameter by using the "trade" size (245/60-16 or whatever). Instead, go directly to the actual thing you want to know and measure it; use a tape measure, and see how far it is from the exact center of your rear wheel, to the ground, WITH THE CAR SITTING ON IT AT NORMAL TIRE PRESSURE. Take that number, multiply by 2pi; that gives you how many inches the car moves per tire rotation. Divide the number of inches per mile, by that number; that will tell you how many times your tires rotate per mile (assuming traction of course!!). Rather than use your gear "ratio" (3.73 or whatever), mulitply that by the number of teeth on your ring gear (41, for a 3.73) and divide by the number of teeth on the pinion (11); that gives you the number of times your driveshaft rotates per mile. (should come out to somewhere around 2500-3000, if you want a "smell test" for your results). Now all you need is to find the closest pair of gear that you can actually buy, to that ratio (1000:2700 or whatever); keeping in mnd that the driven gear is always going to be 7 (that being the only one that actually exists, that will work). So basically you're only going to be able to pick speedo calibration in steps of about 5%, since you can only get whole numbers of driven teeth; 19, 20, 21, 22, etc. Auto trans speedo gears have considerably finer increments; you can typically get them with drive gears of around 14 to 17 teeth, and driven gears from 38 to 48 or thereabouts.

This is definitely one of those "measure with micrometer, mark with chalk, cut with axe" kind of deals.

Let's pluck a number out of the air, and say you come up with 13.25" for your tire radius.

So: 13.25 x 2pi = 83.25etc.; divide 63360 (# of inches per mile) by that, you get 761etc.; multiply by 41 and divide by 11 (your gear ratio), you get 2836.7; divide by 1000 (cable revs per mile), you get 2.8637; multiply that by your drive gear teeth (7), and you discover that you need a driven gear with 19.86 teeth.

Done!!! Was that easy, or what?????

You can only buy a gear with either 19 or 20 teeth, so you need the 20 tooth one. With a 20-tooth driven gear, your speedo will be slightly less than 1% lower than reality; i.e. it will read 59.4 mph when you're really going 60, 99mph when you're really going 100, etc. I suspect that that's plenty close for most of us, most of the time.

See how simple and easy it is? Forget all these pages on the Internet that "give" the "correct" gear combos; among other problems, THEY DON'T TAKE YOUR SPECIFIC EXACT TIRE SIZE into account, and are therefore only accurate if your tires happen to match whatever tire radius they used to derive their gear tooth counts.

Just go measure, and calculate exactly as I did, and DON'T WORRY ABOUT what other people "are running", or what's "correct" according to whatever Internet site, or any of that drivel and stupidity. Just run the numbers FOR YOUR CAR and go with what you get, and it will have no choice BUT to be right.

Then you can check your result by driving along a measured stretch of interstate or something and seeing if the odometer agrees with the mile markers, if you want to QC yourself.
Old 04-10-2006, 09:30 AM
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Car: '89 Firebird Formula
Engine: 305 TBI
Transmission: T5
Tires: 245/50R16
AR (axle ratio): 2.73
T1 (teeth #1): 9 (red gear)

TD (tire diameter): 25.65"
TRPM (tire revolutions per minute): 20168 / TD = 786.28
T2 (teeth #2): T1 * AR * TRPM / 1001 = 19.30
which according to that with a 7 tooth driven gives you 20.5.. so I guess 20 or 21 tooth driven is your options

Drive Gears:
Purple - 7 #14071731
Red - 9

Driven Gears:
Red - 18
White - 19 #14090592
Light Blue - 20
Orange - 21 #14090582
Gray - 22
too late, extra information, disregard ;0
Old 04-10-2006, 01:10 PM
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I run a 7 and a 21.
Old 04-10-2006, 03:11 PM
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Originally Posted by sofakingdom
I'm running 7 and 20 IIRC. Just the stock combo for my car.

Which incidentally, I moved over to my T-56 without incident.

I fail to see how it is possible that anyone with enough intelligence to count their fingers and toes at the same time, would have any difficulty whatsoever figuring this out. It's got to be one of the easiest things about a car to calculate. Easier by far than, say, compression ratio (not that most people who are pretending to calculate that, ever get it right; but that's a different rant..) If I, a total imbecile and moron, can manage it, then what does that say about people who can't?

Recall that your car has an odometer. This is the simplest of all devices. Think about it: all it is, is a turns counter. It counts how many times the cable turns. And, it's hooked up to THE SAME SOURCE OF MOTION as the speedo. Use this to your advantage.

The odometer has a ratio of 1000:1. Which means, 1000 turns of the cable, makes it advance by one mile. Pretty simple, eh? That's how a speedo service can calibrate your speedo when it's out of the car, WITHOUT YOUR SPEEDO GEARS; they know how fast to spin the cable to make it read whatever. 1000 RPM = 60 MPH. How how much more simple and mindless can it get?

So all you have to do is figure out how many thousands of times your driveshaft rotates in one mile, and get a set of speedo gears in that same ratio. That's all there is to it.

Go measure your wheel radius; NOT "calculate" the tire diameter by using the "trade" size (245/60-16 or whatever). Instead, go directly to the actual thing you want to know and measure it; use a tape measure, and see how far it is from the exact center of your rear wheel, to the ground, WITH THE CAR SITTING ON IT AT NORMAL TIRE PRESSURE. Take that number, multiply by 2pi; that gives you how many inches the car moves per tire rotation. Divide the number of inches per mile, by that number; that will tell you how many times your tires rotate per mile (assuming traction of course!!). Rather than use your gear "ratio" (3.73 or whatever), mulitply that by the number of teeth on your ring gear (41, for a 3.73) and divide by the number of teeth on the pinion (11); that gives you the number of times your driveshaft rotates per mile. (should come out to somewhere around 2500-3000, if you want a "smell test" for your results). Now all you need is to find the closest pair of gear that you can actually buy, to that ratio (1000:2700 or whatever); keeping in mnd that the driven gear is always going to be 7 (that being the only one that actually exists, that will work). So basically you're only going to be able to pick speedo calibration in steps of about 5%, since you can only get whole numbers of driven teeth; 19, 20, 21, 22, etc. Auto trans speedo gears have considerably finer increments; you can typically get them with drive gears of around 14 to 17 teeth, and driven gears from 38 to 48 or thereabouts.

This is definitely one of those "measure with micrometer, mark with chalk, cut with axe" kind of deals.

Let's pluck a number out of the air, and say you come up with 13.25" for your tire radius.

So: 13.25 x 2pi = 83.25etc.; divide 63360 (# of inches per mile) by that, you get 761etc.; multiply by 41 and divide by 11 (your gear ratio), you get 2836.7; divide by 1000 (cable revs per mile), you get 2.8637; multiply that by your drive gear teeth (7), and you discover that you need a driven gear with 19.86 teeth.

Done!!! Was that easy, or what?????

You can only buy a gear with either 19 or 20 teeth, so you need the 20 tooth one. With a 20-tooth driven gear, your speedo will be slightly less than 1% lower than reality; i.e. it will read 59.4 mph when you're really going 60, 99mph when you're really going 100, etc. I suspect that that's plenty close for most of us, most of the time.

See how simple and easy it is? Forget all these pages on the Internet that "give" the "correct" gear combos; among other problems, THEY DON'T TAKE YOUR SPECIFIC EXACT TIRE SIZE into account, and are therefore only accurate if your tires happen to match whatever tire radius they used to derive their gear tooth counts.

Just go measure, and calculate exactly as I did, and DON'T WORRY ABOUT what other people "are running", or what's "correct" according to whatever Internet site, or any of that drivel and stupidity. Just run the numbers FOR YOUR CAR and go with what you get, and it will have no choice BUT to be right.

Then you can check your result by driving along a measured stretch of interstate or something and seeing if the odometer agrees with the mile markers, if you want to QC yourself.

I need the part #'s for the T5 gears, I know the ratios..

I was told there is NO 20, 21 for a T5...
----------
Originally Posted by dr1
which according to that with a 7 tooth driven gives you 20.5.. so I guess 20 or 21 tooth driven is your options



too late, extra information, disregard ;0
Are those #'s for a T5 or 700R4 ?

-- Joe

Last edited by anesthes; 04-10-2006 at 03:12 PM. Reason: Automerged Doublepost
Old 04-10-2006, 03:44 PM
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The part #s exist; I just bought one not too long ago. Too bad I didn't keep the receipt.

Mostly we're all having to deal with a buch of lazy parts counter monkeys who only get to sell a $5 part, but it takes them 5 minutes to look it up. They'd rather just blow you off than deal with your hassle. Then they go sit down at their desk and price out a $4000 collision repair. Takes an hour or 2, is probably 15-20 times as profitable in $$$/hr.

Those numbers look like T-5 ones to me. Most of the 700 gears are up in the 17 & 44 kind of ratios, not 7 & 20.
Old 04-10-2006, 03:48 PM
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Try sending jmd an email. Thats where I got mine.
Old 04-10-2006, 04:17 PM
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those were T5 gear #s taken from one of many bookmarked posts on the subject, i can confirm the 7 tooth driven and 19 tooth drive #s as i just had to order them

also I dont know if you use the VSS or mechanical drive or what, but apparently theres more then one set of driven gears
https://www.thirdgen.org/forums/tran...SS+part+number

Last edited by dr1; 04-10-2006 at 04:22 PM.
Old 04-11-2006, 08:15 AM
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Originally Posted by dr1
those were T5 gear #s taken from one of many bookmarked posts on the subject, i can confirm the 7 tooth driven and 19 tooth drive #s as i just had to order them

also I dont know if you use the VSS or mechanical drive or what, but apparently theres more then one set of driven gears
https://www.thirdgen.org/forums/tran...SS+part+number
I'm using the older style firebird VSS, not the optical one on the later birds and camaro's.. Hrmm..


btw, Orange - 21 #14090582 no longer appears available from gmpartsdirect or the dealer.. Wonderful.

Anyone have a part # for a 20 or 21 tooth gear ?

-- Joe

Last edited by anesthes; 04-11-2006 at 08:19 AM.
Old 04-11-2006, 10:48 AM
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My 83 z28 came from the factory with a T-5 and 3.73 gears, see if they can look up that gear combo.
Old 04-11-2006, 11:01 AM
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Originally Posted by todd200
Try sending jmd an email. Thats where I got mine.
Me too.

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Old 04-11-2006, 11:32 AM
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Originally Posted by 89gta383
My 83 z28 came from the factory with a T-5 and 3.73 gears, see if they can look up that gear combo.
Ahhhh.. Just ran 21 tooth for a '83 z28, part # 3987921 .. $5.24, in stock!!

Ordered one and I ordered a 14071731 which is 7 tooth. Should be here tomorrow.

Both 14090582, 14090594 were not available. Infact, both are discontinued as of 10/1999. I'm sure there is NOS lying around here and there, but I couldn't find any online sites or local dealers that had one.

I'll report back if it fits the T5. The guy at the dealership was a little worried that it might be for a 4spd manual, because of the way the part # was. But it appears the only cars that got 3.73's in 1983 was M5 cars, the M4 was only available in a 2.5l with 3.42..


-- Joe
Old 04-11-2006, 11:59 AM
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The same gears fit 4-spds as fit T-5s.

I've got probably a dozen of them laying around, with various tooth counts.
Old 04-11-2006, 12:24 PM
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err I dont want to rain on the parade but you may want to click on the link I put in a few posts back..

with that number it seems your going to end up with an all plastic gear that im pretty sure will have no way to fit on the VSS
Old 04-11-2006, 12:46 PM
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Originally Posted by dr1
err I dont want to rain on the parade but you may want to click on the link I put in a few posts back..

with that number it seems your going to end up with an all plastic gear that im pretty sure will have no way to fit on the VSS
Im confused about that. The gears I have right now are all plastic... ??


-- Joe
Old 04-11-2006, 01:04 PM
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I got one of the 140* gears, plastic gear comes on a metal shaft, then I had to take it off the shaft so it would fit on the non-removeable shaft thats already fitted on my vss.. dunno what I would do with one that came molded onto a plastic shaft
Old 04-12-2006, 08:05 PM
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Originally Posted by dr1
I got one of the 140* gears, plastic gear comes on a metal shaft, then I had to take it off the shaft so it would fit on the non-removeable shaft thats already fitted on my vss.. dunno what I would do with one that came molded onto a plastic shaft
Yep, it's the wrong gear. oh well.

I have a 7 tooth in the tranny. (checked tonight), a 18 tooth, and a 22 tooth. The 22 tooth reads 10+ mph too fast..
Old 04-12-2006, 11:11 PM
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20 Blue-----(14090593)

give that a try, and if you want to offload that 18 tooth, id take it off your hands, i might need one

hmm nevermind, everything i read seems to say 20,21 are discontinued all together heh.. i think 19 & 22 are the only gears you can even get ahold of, its these stupid little peices I wish someone would just start producing, send me some gears ill make some molds and stockpile them ;0

Last edited by dr1; 04-12-2006 at 11:31 PM.
Old 04-13-2006, 06:27 AM
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Originally Posted by dr1
20 Blue-----(14090593)

give that a try, and if you want to offload that 18 tooth, id take it off your hands, i might need one

hmm nevermind, everything i read seems to say 20,21 are discontinued all together heh.. i think 19 & 22 are the only gears you can even get ahold of, its these stupid little peices I wish someone would just start producing, send me some gears ill make some molds and stockpile them ;0
The 18 would work if I could find a 6 tooth. But that won't happen.. This kinda blows.

How do I contact JDM ?

-- Joe
Old 04-13-2006, 07:58 AM
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Originally Posted by anesthes
The 18 would work if I could find a 6 tooth. But that won't happen.. This kinda blows.

How do I contact JDM ?

-- Joe

let me know if he ever gets back to you. I have tried for a long time but, he never replys


I made my own out of two gears but, I think I would rather have the right one
Old 04-26-2006, 11:08 AM
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20 and 21 for the VSS (comes on a metal shaft) are discontinued. I last bought some of them last year and am sold out.

I have 19 through 25 for cable driven speedos currently but no more of the 20 and 21 for VSS speedos.
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