Speedometer DRIVEN Gear removal
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
Speedometer DRIVEN Gear removal
My speedometer reads about 10 - 15% faster at all speeds. ~75 when actual speed is 65. Can't tell exactly, because speedo cable also needs to be replaced, needle jumps 10 and 15 mph above and below actual speed, so I've taken an average and occasionally it calms down and is steady long enough to get a decent read.
I am trying to get to the driven gear in the transmission to determine how many teeth it has, but I'm having problems removing the fitting (don't know the actual name) that holds the gear in. I know it pulls straight out after removing the hold-down bracket, but I can't get a good grip on it. I can spin it by leveraging a screwdriver against the tabs, so it's not stuck in there. Any ideas?
I've Googled this and anything I've found is too vague or refers to the drive gear inside the transmission.
Also, will any transmission fluid come out when I do get it out?
TIA!
Tom
I am trying to get to the driven gear in the transmission to determine how many teeth it has, but I'm having problems removing the fitting (don't know the actual name) that holds the gear in. I know it pulls straight out after removing the hold-down bracket, but I can't get a good grip on it. I can spin it by leveraging a screwdriver against the tabs, so it's not stuck in there. Any ideas?
I've Googled this and anything I've found is too vague or refers to the drive gear inside the transmission.
Also, will any transmission fluid come out when I do get it out?
TIA!
Tom
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
Unfortunately, that's exactly what I've been trying to do, but it won't budge outwards more than a millimeter or two. Even tried reattaching the speedo cable to get a better grip. nothing...
Any thoughts on how much fluid may come out when it is removed? One video I found showed about a pint coming out. I'm under the car and it's on jack stands, so I don't have much room to jump out of the way and I don't want a shower of transmission fluid!
Le Mans Master
Joined: Apr 2001
Posts: 5,941
Likes: 281
From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Thanks for identifying it for me, I would never have guessed to call it that!
Unfortunately, that's exactly what I've been trying to do, but it won't budge outwards more than a millimeter or two. Even tried reattaching the speedo cable to get a better grip. nothing...
Any thoughts on how much fluid may come out when it is removed? One video I found showed about a pint coming out. I'm under the car and it's on jack stands, so I don't have much room to jump out of the way and I don't want a shower of transmission fluid!
Unfortunately, that's exactly what I've been trying to do, but it won't budge outwards more than a millimeter or two. Even tried reattaching the speedo cable to get a better grip. nothing...
Any thoughts on how much fluid may come out when it is removed? One video I found showed about a pint coming out. I'm under the car and it's on jack stands, so I don't have much room to jump out of the way and I don't want a shower of transmission fluid!
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Get ready. Have a deep pan at your disposal. Lots of towels. Because over a half a teaspoon may come out. 99.99% of the ATF is either in the torque converter or in the pan. There is hardly any in the tail shaft housing.
You may have to grab the bullet with some channel-locks. Obviously the "O" ring is fighting you and I hope you have a spare.
Don't let the driven gear fall back inside the casing. Not fun fishing for that.
There are charts on the net to help you find the almost perfect new gear. Rear axle ratio, tire size all come into play. With your 10 MPH error, you may need two more teeth?
You may have to grab the bullet with some channel-locks. Obviously the "O" ring is fighting you and I hope you have a spare.
Don't let the driven gear fall back inside the casing. Not fun fishing for that.
There are charts on the net to help you find the almost perfect new gear. Rear axle ratio, tire size all come into play. With your 10 MPH error, you may need two more teeth?
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
Get ready. Have a deep pan at your disposal. Lots of towels. Because over a half a teaspoon may come out. 99.99% of the ATF is either in the torque converter or in the pan. There is hardly any in the tail shaft housing.
You may have to grab the bullet with some channel-locks. Obviously the "O" ring is fighting you and I hope you have a spare.
Don't let the driven gear fall back inside the casing. Not fun fishing for that.
There are charts on the net to help you find the almost perfect new gear. Rear axle ratio, tire size all come into play. With your 10 MPH error, you may need two more teeth?
You may have to grab the bullet with some channel-locks. Obviously the "O" ring is fighting you and I hope you have a spare.
Don't let the driven gear fall back inside the casing. Not fun fishing for that.
There are charts on the net to help you find the almost perfect new gear. Rear axle ratio, tire size all come into play. With your 10 MPH error, you may need two more teeth?
Yes! I finally got it out! I ended up using channel locks and reinstalled the old speedometer cable to grab onto. It actually took quite bit of effort, I was surprised. A little over a cup of fluid came out when I pulled it, so at least it wasn't a full on waterfall, like the video I saw.
The driven gear that's installed has 35 teeth.
I'm trying to figure out if it's 2 or 3 teeth extra I need. Some sites say 3.3mph per tooth. Some say 5mph. I think it depends on whether its an automatic or manual transmission. From what it looks like on sites like Corvette Central or Top Flight Auto, the manual transmission speedometer driven gear is smaller than the one for an automatic. That could just be forced perspective when they took the photo, though the number of teeth are fewer on the manual. Which proportionally should mean that it would have a greater affect on the MPH, i.e. 5 mph.
Any thoughts?
Le Mans Master
Joined: Apr 2001
Posts: 5,941
Likes: 281
From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Thanks for the help!
Yes! I finally got it out! I ended up using channel locks and reinstalled the old speedometer cable to grab onto. It actually took quite bit of effort, I was surprised. A little over a cup of fluid came out when I pulled it, so at least it wasn't a full on waterfall, like the video I saw.
The driven gear that's installed has 35 teeth.
I'm trying to figure out if it's 2 or 3 teeth extra I need. Some sites say 3.3mph per tooth. Some say 5mph. I think it depends on whether its an automatic or manual transmission. From what it looks like on sites like Corvette Central or Top Flight Auto, the manual transmission speedometer driven gear is smaller than the one for an automatic. That could just be forced perspective when they took the photo, though the number of teeth are fewer on the manual. Which proportionally should mean that it would have a greater affect on the MPH, i.e. 5 mph.
Any thoughts?
Yes! I finally got it out! I ended up using channel locks and reinstalled the old speedometer cable to grab onto. It actually took quite bit of effort, I was surprised. A little over a cup of fluid came out when I pulled it, so at least it wasn't a full on waterfall, like the video I saw.
The driven gear that's installed has 35 teeth.
I'm trying to figure out if it's 2 or 3 teeth extra I need. Some sites say 3.3mph per tooth. Some say 5mph. I think it depends on whether its an automatic or manual transmission. From what it looks like on sites like Corvette Central or Top Flight Auto, the manual transmission speedometer driven gear is smaller than the one for an automatic. That could just be forced perspective when they took the photo, though the number of teeth are fewer on the manual. Which proportionally should mean that it would have a greater affect on the MPH, i.e. 5 mph.
Any thoughts?
https://www.tciauto.com/speedometer-gear-calculator
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Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
It will be tough to get the speedometer exact. So, if you have to error on accuracy, always error on the high side.
If your speedo reads 70 and you are actually driving 65-66, no harm no foul. But if you are actually driving 75+ that is what will get you in trouble.
You will only be allowed to change the driven gear a certain set number of teeth. Beyond that maximum you will have to change out the Drive Gear. Not a horrible job, but one that I only want to do once.
Calculations might be available at the web site above. Just guessing, I think two more teeth should be error on the high / safe side at 6.6 MPH increase.
If your speedo reads 70 and you are actually driving 65-66, no harm no foul. But if you are actually driving 75+ that is what will get you in trouble.
You will only be allowed to change the driven gear a certain set number of teeth. Beyond that maximum you will have to change out the Drive Gear. Not a horrible job, but one that I only want to do once.
Calculations might be available at the web site above. Just guessing, I think two more teeth should be error on the high / safe side at 6.6 MPH increase.
Burning Brakes
Joined: Apr 2020
Posts: 783
Likes: 415
From: Western NY
The most teeth you can get is 45 on the DRIVEN gear. depending on how many teeth your DRIVE gear has will determine how many mph's your speedo will change per tooth of DRIVEN gear change. there are 3 or 4 different gears available for the DRIVE gear. they are in the 15-18 teeth range. either way i doubt your speedo will change anywhere near 5mph per tooth but i could be wrong. just for real world info here is what i have and this is verified by me wanting my speedo to be correct. i changed my gears to 3.70 and have 255/60's on the back. In my TH400 i have the drive gear with 15 teeth. for every increase or decrease in driven gear teeth my speedo changes 1.5mph. if you don't want a supply of driven gears on the shelf like i have i would suggest purchasing a gear with 38 or 39 teeth a find out what it does for you then you can make a fairly accurate determination of what gear you need to get your speedo correct. also keep in mind there are 2 gear housings for the various gears. one housing is for 35-39 teeth and the other is for 40-45 teeth - i am typing this off the top of my head so i may be off by 1 tooth on the housings.
Pat
Pat
Safety Car
Joined: Mar 2009
Posts: 4,191
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From: Corpus Christi, TX
Get the gear that gives you a correct odometer reading on a measured mile run of say 10 miles. Once that's correct, then deal with the speedometer. 10 to 15% error on a 35 tooth gear is 3 to 5 teeth more needed on the driven gear.
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
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From: Benicia, CA
I am resurrecting this thread from a few months ago. I received the new driven gear and went to put it in the transmission a month or so ago (then a more pressing issue elsewhere took my attention away), when installing it, there doesn't feel like there is any support to hold up the gear inside the transmission. When I previously pulled the old one out and this time as well, it felt like there is something supporting it besides the housing bullet. The old and new are identical in shape and size and have the same holes at the ends of the spindles.
Is the only thing keeping the gear in place the bullet housing and speedometer cable? Or should there be a small "pin" or protrusion inside the transmission that supports the far end of the gear?
Is the only thing keeping the gear in place the bullet housing and speedometer cable? Or should there be a small "pin" or protrusion inside the transmission that supports the far end of the gear?
Safety Car
Joined: Oct 2004
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From: Youngstown Ohio
Last edited by C3 Stroker; Oct 15, 2023 at 09:01 PM.
1967 Pedal Car Champion
Joined: Mar 2020
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From: US-PNW
What is your tire size? Original tires were just shy of 27.5" diameter; larger or smaller diameter will affect the speedometer reading.
A bouncing needle could be a situation where excessive cable lube has made its way to the speedo head. It could also be a binding cable in the sheath. Connect a drill to the transmission end of the cable and spin it up (reverse, I believe) while observing the needle. If it still bounces this will eliminate the transmission portion.
A bouncing needle could be a situation where excessive cable lube has made its way to the speedo head. It could also be a binding cable in the sheath. Connect a drill to the transmission end of the cable and spin it up (reverse, I believe) while observing the needle. If it still bounces this will eliminate the transmission portion.
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
Great! Thanks! I was trying to just insert the driven gear into the transmission and then place the bullet. Makes sense now that you said it. I guess that the gear just rests against the inside wall of the transmission and that's what holds in place.
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
What is your tire size? Original tires were just shy of 27.5" diameter; larger or smaller diameter will affect the speedometer reading.
A bouncing needle could be a situation where excessive cable lube has made its way to the speedo head. It could also be a binding cable in the sheath. Connect a drill to the transmission end of the cable and spin it up (reverse, I believe) while observing the needle. If it still bounces this will eliminate the transmission portion.
A bouncing needle could be a situation where excessive cable lube has made its way to the speedo head. It could also be a binding cable in the sheath. Connect a drill to the transmission end of the cable and spin it up (reverse, I believe) while observing the needle. If it still bounces this will eliminate the transmission portion.
I have replaced the cable and made sure it makes all the turns as smoothly as possible. It's just one long cable, as there is no cruise control. I would not doubt that the PO over lubed it with actual grease rather than graphite, but when I had the dash apart earlier in the year, I attached a drill directly to the back of the speedometer and turned it at up to about 25-30 mph and there was no bounce in the needle. The old transmission driven gear was undamaged, is it possible the drive gear in the transmission could be damaged?
Thanks for the help!
Melting Slicks
Joined: Feb 2018
Posts: 2,319
Likes: 720
From: Marietta GA
I had the OEM speedo and tach rebuilt by a extremely competent shop. Even with a new cable my speedo is about 8 mph off. The manual tranny gear along with a new cable was replaced about a year ago. If you want an exact speedometer reading, may I suggest a C8, they certainly don’t use 50 year old technology … gears, and a cable, with a couple of bends. I would also venture to say the mechanical tach in my 73 is not that accurate.
Thread Starter
Instructor
Joined: Feb 2023
Posts: 159
Likes: 31
From: Benicia, CA
I had the OEM speedo and tach rebuilt by a extremely competent shop. Even with a new cable my speedo is about 8 mph off. The manual tranny gear along with a new cable was replaced about a year ago. If you want an exact speedometer reading, may I suggest a C8, they certainly don’t use 50 year old technology … gears, and a cable, with a couple of bends. I would also venture to say the mechanical tach in my 73 is not that accurate.
I'm not looking for exact, just in the ballpark. I'm sure even with the new gear it will still be 2-3% off, but that's better than 10%
Actually the tachometer on mine is fairly accurate, compared it with the digital readout on my timing gun. However, when the tach/cable/gears gets too hot, after a long drive or sitting after being out, it begins working intermittently. Just drops to zero, then 5-10 minutes later pops back up.
Ahh, the joys of classic car ownership!