bought my 79 supercab a couple of months ago and the speedo didn't work. I pulled the cable out of the transfer and there was no gear there. I looked in the hole with a flashlight and there was the gear, not what I wanted to see. I left it alone and have just been keeping up with traffic but today I decided to pull the tail housing off and see what was going on. Well what I found was interesting (to me anyway). once I pulled the driveshaft and then the rear yoke and the first level of tail housing and still couldn't see the gears so I went to the next level of the tail housing. As I pulled it off the rear output shaft I began to notice that there was a lot of stuff in the bottom of the housing.

Here is the tail housing (I may be calling it the wrong name)


I found all of these in the bottom of the housing


with this clip


looks like who ever worked on it last didn't get the c-clip on the cable gear very well and so it came off and fell into the bottom of the housing.

 

next I found that this gear was not in good shape. You can see that the middle of the gear is pretty worn out.


I assume that the neddle bearings go in here? a little grease ought to hold them in while I reinstall them.


A quick trip to the local Ford Dealer and $42 latter I came home with this.


in the morning I will begin to put it all back together and then get a new speedo cable gear and c-clip.

What I need help with is that since the truck never had a working speedo I don't know how far off it would have been. And I have put larger tires on since then as well. My current setup is 4.10 rear gears, C6, NP205, 315-75R16's. What Gear color should I start off with or could somebody tell me exactly what the right gear would be. love this site and thanks for the help in advace. I will post more Pics as I put it all back together.

Sam

 

Well, I got it back together with all the bearings back in place and new gaskets. I found that the speedo cable was to long in the sleeve and that whoever put it in hadn't trimed it to the proper length and so the gear and clip had come off. I trimed the cable and installed a gear i found in the glove box. it is a light pink or maybe cream color but the speedo works like a charm though I am sure that it is off by more than 10% slow so i will run it with the GPS tomorrow and see how far off it really is and then begin to figure out what gear I really need.

more to come

 

my truck is in a similar way. the lights didn't work, i replaced the whole speedo, and found a worn gear, replaced and it has worn again. sometimes the speedo works and sometimes not.

 

The speedo driven gears are still available from Ford. 15 thru 21 teeth are offered.

Driven Gear = on the cable. Ford basic part number 17271.

Drive Gear = on the output shaft. Ford basic part number 17285.

 

If you can figure out how far off your current speedo & gear are, it's easy to get the right one. Lets say you are going an indicated 60, a radar speed trap says you're going 66. That means you have a 10% error, the speedo is 10% slow. So, you need to change the count on the teeth by 10% & you should be close to accurate. To slow it down you need more teeth, to speed it up you need less teeth. In this example lets say you have an 18 tooth gear. You need 10% less teeth, or a 16 tooth gear.

Doing this is much easier than guesswork, especially when tire sizes are changed.

The police in my area have these small trailers with radar mounted on them. The police park them along the roads, I guess to slow people down. (its like a game to some, how high of a score can I get?) They show you how fast you're going when you pass. I use those to see what I need to change. You can also drive alongside someone with a known accurate speedo & calculate the error from the difference between the two.

Try www.lowpricegears.com for new gears. Scott

 

Rockdawg,

I have a similar problem, have to replace the drive gear in the 205. How did you get the yoke nut off? I have the right size socket and tried using an electric impact driver last night but it would not budge. Any hints?

Thanks

 

Mine came off really easy. but here is my suggestion for what it is worth. Take a breaker bar and put it on your socket. The put a long piece of pipe over the breaker bar (long enough to hit the frame. then start your truck and put it in drive or reverse I'm not sure which and use the torque of the engine to loosen the nut. Be really careful and don't have anyone under the truck or anything but that should get it loose. Good luck.

thanks I drove the truck today and found that my speedo is off about 27 percent. At 50mph on my speedo the GPS says 63.5mph. So I will crawl under and pull the cable gear and count the teeth and hopefully they will sell one the will work. thanks for all your help guys

 

thanks Bill, I got the drive gear from the dealer and it was part number 17285. Had to look on the package picture in my post it see. It wasn't even in their computer, we just matched my old one to the new one they had in stock LOL. Finding the right driven gear may be fun but no biggie. again thanks.

Sam

 

I think there is also an 8 tooth drive gear. As far off as your speedo is, you may need to change to one of those..... : ( From the pictures it looks like you have a 7 tooth drive gear. Scott

 

well I pulled the driven gear and found that it was an 18 tooth. that means I tecnically would need a 13 tooth gear but as Bill said they only go down to 15 and so I don't really know what to do. Bill, do you have a part number on the 8 tooth drive gear? I really want my speedo to be accurate.

thanks

sam