Speedometer Gear Change
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Speedometer Gear Change
Posted: November 10 2007 12:42 AM
 
Leo Gengler (aka roofcam34) [ View ] [ leogengler@msn.com ] [ Car Ads ] [ Blogs ]
Sioux City, Iowa
(712) 899-7440
 Leo (roofcam34)
 
Newbie needs help. Anyone know how tough it is to remove the steel Speedo drive gear from the output shaft (it's pressed on) on a TH400, and if it requires a special tool? My MPH is about 20 MPH off from actual, so need some changes. Thanks.
 
 
Comments
 
Posted by DavyJ  -  11/10/2007 01:08 AM
Try a reducer off an overdrive tranny, they look like a small box which screws on to the tranny and then the speedo cable screws into it. if it takes the speed the wrong way, you just split it open and reverse the gears inside. i have found these on 80's S10's
Still driving the wife's car, cause I am slow. [ View ] [ Email ] [ Blogs ] [ Car Ads ] Niagara Falls, Canada Charter Member since February 2005
 
Posted by JAWS  -  11/10/2007 03:09 AM
yep
An electrical headache can make a great car a pain [ View ] [ Email ] [ Blogs ] [ Car Ads ] Boise, Idaho Charter Member since August 2004
 
Posted by Italianguy63  -  11/10/2007 08:16 AM
This should help-- look for the calculator link in the middle of the page. MC


http://www.73-87.com/7387garage/drivetrain/speedo.htm
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
Posted by jimmothershead  -  11/10/2007 01:59 PM
I have some of those "boxes" that DavyJ refers to. They change the gear ratio. Want one?

You do have a mechanical speedo, right?

No problems, no fun with rods. Jim, 410 535 1933, 410 535 1932
Jim [ View ] [ Email ] [ Blogs ] [ Car Ads ] Huntingtown, Maryland
 
Posted by ol bear  -  11/10/2007 04:17 PM
THATS WHTY I WENT WITH A CLASSICS INSTRUMENTS GAUGE PACKAGE , TAKE S THE WORRY OUT OF BEING CLOSE ,, JUST LIKE IN THE SURE DEODARANT COMMERCIAL ,,,,,,,,,,,OL BEAR
OL BEAR [ View ] [ Email ] [ Blogs ] [ Car Ads ] crown point, Indiana
 
Posted by BigAlBre  -  11/10/2007 04:34 PM
It was real easy in my 2004R.... went down to the local transmission place to get a new gear.... note be careful where the crimped area for the cable is.... mine wouldn't reach.... so I got one that had the crimp at the end of the gear shaft....
Big Al sittin back in paradise. [ View ] [ Email ] [ Blogs ] [ Car Ads ] St Pete Beach, Florida Charter Member since June 1999
 
Posted by roofcam34  -  11/11/2007 01:43 AM
jimsnice car.......I've heard that the box will only work for up to 10% off. True? Mine is waaay more than that.
Leo (roofcam34) [ View ] [ Email ] [ Blogs ] [ Car Ads ] Sioux City, Iowa
 
Posted by roofcam34  -  11/11/2007 02:25 AM
Italianguy63...............Thanks for the post. However, I have the gears/teeth I need figured out. What I'm trying to determine, is how the "pressed on" drive gear on my TH400 output shaft is removed, i.e., is a special puller required? Unfortunately, it is not like some of the trannys that only need a clip removed to replace the drive gear. Maybe I best leave this task to a transmission shop. Thanks to all that have provided information.
Leo (roofcam34) [ View ] [ Email ] [ Blogs ] [ Car Ads ] Sioux City, Iowa
 
Posted by Italianguy63  -  11/11/2007 04:05 AM
Roofcam-- I haven't had the experience of running into that before. How far off is your ratio? And WHY? How many teeth does your "drive" gear have? You can't get it close using a particular "driven" gear? Double check your calculations with me, Would you believe I had a TH400 rebuild manual on my shelf for like the last 10 years and JUST gave it away about a month ago? I would hate to see you have to pay a tranny shop for something that should be relatively easy. MC
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
Posted by maniac  -  11/11/2007 04:53 PM
My TH 400 was 40 miles off when i first put it in, I called United Speedometer in Cali and they told me what to do to determine which gear I needed, marked off a certain distance in the street, than counted the turns of the speedo cable, in the set distance, when I got the figure, the guy told me which gear to buy, they were very helpful.

The gear in the trans needs to be heated just slightly to get it off, kind of like doing a wrist pin, but you shouldn't have to mess with it at all, TH 400's were used in some trucks where the ratio was as high (low) as 5:38. Changing the plastic gear should be all you need to do.

maniac [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bethel, Connecticutt
 
Posted by Italianguy63  -  11/11/2007 05:19 PM
Right-- Roofcam, give me your tire size or diameter, your rear end ratio, and how many teeth you count on the gear on the shaft, and I will look it up for you.


Mark
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
Posted by Jaybird  -  11/11/2007 06:59 PM
There is also a calculaor on www.bowtieoverdrives.com plug in tire diam, etc and it tells you what speedo gears you need to bring things into spec
Jaybird [ View ] [ Email ] [ Blogs ] [ Car Ads ] Cincinnati, Ohio Charter Member since December 2003
 
Posted by roofcam34  -  11/12/2007 02:21 AM
Ok Guys. Although I said I have the necessary gear replacements figured out, many keep asking to calcualte the need. So, here's the info. This is in a streetrod, with a 350 engine, a TH400 tranny, and a Ford 8.8 rearend. The tires are exactly 28" diameter, and the rearend ratio is 2:73. novaresorces.com and the other calc sites turn all of this into a need for one of 3 combos to get me on or very close to accurate. They say the gears I need are one of these 3 combos: 17T & 34T; 18T & 35T; or 18T & 36 T. The current driven gear is 40T. I don't feel that changing the 40T gear alone will even get me close. The next thing I need to do, is remove the tranny tail housing and count the teeth on that existing steel speedo drive gear. I'll get my ducks lined up. I appreciate all your advice and suggestions!! This a great site. Thanks.
Leo (roofcam34) [ View ] [ Email ] [ Blogs ] [ Car Ads ] Sioux City, Iowa
 
Posted by Italianguy63  -  11/12/2007 12:38 PM
YES. Those calculations are in the ballpark! Count the teeth on the shaft! Then replace the driven gear-- I can get you part #'s easily for what you need. But, there are 2 aluminum sleeves that hold that gear: one supports 30-39 teeth, and the other one is for 40-45 teeth. You will have to change out that sleeve too. It is not hard. This will be an inexpensive fix. P.S. you can remove the sleeve, and mark the gear in the tailhousing with a marker and turn the driveshaft to count the teeth!

Mark
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
Posted by roofcam34  -  11/12/2007 04:20 PM
Mark....I will use your method to mark that drive gear. That should be much lees work than removing the tail housing. Thanks for that!! The sleeve in the tranny is the 40-45 tooth one. I do have another that is marked for 34-39 tooth gears, rather than the 36-39 you mentioned. I'd like to be lucky enough to be able to just change the driven gear, utilizing the existing drive gear. I will let you know what I find. Thanks again.


Leo
Leo (roofcam34) [ View ] [ Email ] [ Blogs ] [ Car Ads ] Sioux City, Iowa
 
Posted by Italianguy63  -  11/12/2007 10:43 PM
No problem-- count it and I can give you the GM part# for the driven gear. You can get it cheap online from GM or I probably even have a link where you can get a used one for like $5.


MC
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
Posted by roofcam34  -  11/18/2007 11:54 PM
Mark.....The drive gear has 15 teeth. My calculations indicate that I need one with 17 teeth for use with the 34T driven gear in order to get the speedo to be accurate. The sleeve I have actually shows it is for 36-39 tooth gears. GM told me it would work with a 34T, so I installed it. It brought me much closer, but with the 15T driven gear, it likely is not yet where it needs to be. I'll do a check on it in a few days to see where I'm at. I haven't seen any gears less than 34 T in the 1-1/2" size. A 33T or 32T would probably put me VERY close. Anyone know if they are available? And if so, would it work with the 36-39 sleeve? Thanks to all of you for your comments and help! And your patience! I bought this car built, from a long distance from home. It was a great buy, but there are few things the seller wasn't honest about. Gives me something to tinker with, eh?
Leo (roofcam34) [ View ] [ Email ] [ Blogs ] [ Car Ads ] Sioux City, Iowa
 
Posted by Italianguy63  -  11/19/2007 07:11 AM
They make a 34 tooth gear-- you are in business. Go to this link and go about 2/3rds down. Look for item #68-1

http://www.400raptor.com/

$11.00, or you can get a different drive gear from the same place and get it exact.

I also compiled this info. some years back for my Jaguar project I did-- http://www.jag-lovers.org/lumps/x/speedo.htm You can go there and get the GM part numbers for everything if you would prefer to get the parts from your stealership.

Mark
Speed is about how fast do you want to spend! [ View ] [ Email ] [ Blogs ] [ Car Ads ] Bartow, Florida
 
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