Speedometer gears - Powerglide vs Turbro-hydramatic - Team Camaro Tech
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post #1 of 12 (permalink) Old Jul 29th, 99, 10:56 AM Thread Starter
 
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I just installed a 350 turbo-hydramatic trans in place of the powerglide in my 68 Camaro. The powerglide has a plastic cable speedometer gear and a steel output shaft gear. The turbo-hydramatic has plastic cable and output shaft gears. Can I use the plastic powerglide cable gear in the turbo to get the speedometer to read correctly? Some people say they are a matched set and cannot be mixed. Thanks for any help.
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post #2 of 12 (permalink) Old Aug 1st, 99, 05:25 PM
 
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iam doing something similar on my 67 and have investigated it alot .. basically the only promblem i was told i could encounter was a shorter life span on the gear or cable no real installation promblems but autometer make a cable and gear set that comes with there speedo's the could replace it. if you are keeping the orig. gauges
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post #3 of 12 (permalink) Old Dec 21st, 99, 02:11 PM
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Guys, I did the same swap and my speedometer is way out of wack. Can you all let me know what size gears you are using in the TH 350.
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post #4 of 12 (permalink) Old Dec 21st, 99, 09:42 PM
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The TH350, depending on the vintage, can have different output gears (the one on the output shaft). These vary in tooth number and hence diameter....

You really need to go with a matched set...or try it and find out how far off your speedo is...then adjust speedo gear to compensate...

Good luck....

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'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver

[This message has been edited by mccorry (edited 12-21-1999).]
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post #5 of 12 (permalink) Old Dec 24th, 99, 11:25 PM
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this will be a long reply!!!
there are 4 Drive gears available:
1) Black - 1.880" (2) White - 1.830" (3) Green - 1.816" (4) Black - 1.802" There are 6 SPEEDO gears avail. Identify these by both COLOR and Spline Number (teeth). Here is how to calculate the proper SPEEDO gear change needed: While driving exactly 55mph, have someone with a stop watch count the exact seconds to pass one mile. If the Drive/Speedo ratio in the trans is correct it will take 65 seconds. If it takes as an example 61 seconds then you need to purchase a new Speedo gear with 2 LESS TEETH. Do the measurement and post it and I will tell you the exact gear to buy. The new gear will run about $7.50. If you change tire sizes this is also the method to "re-set" your speedometer for the new tire size. Regards and good luck.
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post #6 of 12 (permalink) Old Dec 26th, 99, 04:06 PM
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Thanks for the information Travis but it doesn't answer my question. The problem I have is when I try to calculate the number of teeth I need, it comes out to a gear that does not exsist. Therefore I need to know, what gear is used on the transmition shaft of a TH 350 and 2.73 rear end. I know it's one of the four available, just which one will get me close so I can do the calculation. As it stands now my best guess is I'm doing 60 mph when my speedometer is reading 110 mph.
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post #7 of 12 (permalink) Old Dec 26th, 99, 05:26 PM
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RickB1B: the drive gear for your trans is based I believe on the OD value of your output shaft. If you still have yours on the shaft then I believe the stock speedo gear is the Blue-20 tooth gear. However, unless you are running with the same outer dimension tire as the car came with stock then you will have to do the speed/distance check - no way around it. If you dont have the drive gear, then "mic" it and start with that. If you do buy a new drive and have to buy a speedo gear at the same time I dont really know what to tell you! Good luck, just trying to help!
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post #8 of 12 (permalink) Old Dec 27th, 99, 07:13 PM
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Does any body know how many teeth are on the power glide speedo drive gear???
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post #9 of 12 (permalink) Old Dec 28th, 99, 04:47 AM Thread Starter
 
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The speedo gear on my 68 Camaro powerglide with 2:56 rear had 20 teeth. When I switch to the 350 turbo the speedometer read 55 mph when doing 65 mph. The 350 turbo speedo gear had 22 teeth so I put the gear from the powerglide that had 20 teeth in the 350 turbo and it decreased the difference down to 5 mph. The speedometer now reads 60 mph when doing 65 mph. My next step is to go to the local Chev dealer and buy a matching set for my 2:56 gears. You may want to try the same thing for your 2:73 rear end. Good Luck. Let me know how it turns out.
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post #10 of 12 (permalink) Old Dec 31st, 99, 06:10 AM
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g5067, I had this exact same problem in my car. I have a '67 Camaro that was originally a 6 cylinder with a powerglide and it still has the original 6 cylinder 2.73:1 ten bolt rear end. When I bought the car, the speedo was not hooked up cuz the gears were all wrong. The car has a 350 and Th-350 with non original 15" tires now. It took many trips to the local chevy dealer and several gears before I got the right one. You MUST replace the gears with a matched set and the metal output shaft PG gear WILL NOT fit on the output shaft of the TH350. I tried it. But all hope is not lost. You need to go to your local chevy dealer and get somebody that is somewhat helpful in the parts department. They have a book that crossreferences all your car's specs and tells them which speedo gears to order you. Give them the transmission type, the rear end ratio, and rear tire size and they can look up the gear. You'll probably have to hound them for them to do this for you, since these guys don't like to flip through books. It turned out that for my car's mismatched specs, I needed a gearset out of a 74 Camaro or something. But, the speedometer is as accurate as can be now. Good luck to you.
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post #11 of 12 (permalink) Old Jan 2nd, 00, 10:32 AM
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Well I'd just like to update everyone since you all have been so helpful and maybe help someone elst who is having the same problem. I originally had the white 1.830 driving gear in the trans. The speedometer was off by about 20% (this was also using the largest driven gear available 24 teeth). I figured I would need a very large change so I went to the 1.802 driving gear. Now the speedometer is off by about 10% in the other direction. Therefore the 1.816 gear should be just about right. what have I learned with this? First, it gets expensive if you have to do like I did, which is try each of the four driving gears until you find the right one. Second, each of the four driving gears will change the speed of the speedometer by about 20% and the each of the driven gears will change the speed by 1.5-2.0%. So if someone else has this problem try to figure your actual vs indicated speed and use the above numbers to get you close then do the speed test to determine which driven speedo gear to use.
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post #12 of 12 (permalink) Old May 15th, 00, 04:10 PM
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My Speedo is reading around 30% too high. Currently my Drive gear has 10 teeth (1.86" od)while the driven (speedo) gear has 20 teeth. Since they don't make a driven gear big enought for what I need (I need 26 teeth), It looks like I'm going to have to change the drive gear.

It is a TH350 tranny. Anyone wanna give me some idea of whats involved? Thanks.....

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Steve McCorry - Central Ohio Camaro Club
'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver
click here to see my car ---> www.mindspring.com/~mccorry

[This message has been edited by mccorry (edited 05-15-2000).]
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