Okay, I'm trying to figure out why my speedometer doesn't work. I take the cable off the transmission and find that two of the teeth on the plastic drive gear are ground down. Great, that explains why it's not ever worked. So, all I got to do now is buy a new gear. I've got a '53 F100 with the standard 3-speed transmission. When I go online looking for a replacement, I find that the gears come with 19, 20, 21, or 22 teeth. All say that they are for my make and model with the standard 3-speed standard transmission. I counted the teeth on what I have and see that my gear has 20 teeth. But, I have never ever had a working speedometer. And, I cannot say with any certainty that the 20 tooth gear is the right one. How is someone supposed to figure this out? The factory Service Manual says that the drive gear part number is 173322. I cannot find any direct reference to this part. What I find available is part number 17322-A has 20 teeth, 17322-B has 21, 17322-C has 19 and 17322-D has 22. (For some reason the new part numbers to not use one of the "3"s found in the Service Manual number.) And they are all, so they say, for the same year and same transmission. Any and all advice is welcome.

 

The speedometer drive gear needs to match the rear axle ratio and rear tire size. What ratio and tires does your truck have?

 

I know almost nothing about my differential. I have always assumed that it was the basic differential for the year and model. That being so, I guess it's an Integral Housing Hypoid. I cannot find any reference that I can make sense of to rear axle gear ratios in the Service Manual. What I take from your comment is that the speedo drive gear would be changed over if, say, the tire size was increased. My concern had been that perhaps the reason the plastic drive gear got chewed up had to do with the number of teeth on that gear being incorrect for the transmission. But if drive gears are meant to be swapped out to compensate for such things as tire sizes, then it looks like my assumption is wrong, and I should look for another reason that the drive gear lost a few teeth. From what I can tell, 20 teeth appears to be standard. If I ever get my speedometer working, and if the mph reading on the speedo isn't entirely accurate, I'll learn to compensate. Right now, I'd settle for just seeing the needle move.

 

This may make your life a little easier. Instead of doing all the math it does it for you. You only need to know a couple of things about your truck.

http://www.sccoa.com/faq/speedgr.html

 

Your axle ratio, provided it's still original, should be noted on the placard on your glove box door. Being we have similar 215/3 spd '53s I'll guess you have the same 3.92 I do. I had the same predicament and used the white gear (a quick call to a vendor should sort that) and with 215/75R15 tires it's pretty close, according to the roadside radar trailers. Your old gear probably succumbed to a binding cable which, by one of your recent posts, I assume you just replaced (and hopefully lubed before installing.) I can pull it out and count the teeth tomorrow, if you'd like.

 

I was at a HS Soccer tournament today and just got back. So I wasn't able to tell you about the routing of the speedo cable. I will get to it though, but that one poster on the other thread was right about it going under the tranny cover at a little raised area.

Now about your speedo. You should take your instrument panel out, if it isn't out now. I learned just last week that it can come out through the front of the dash. When I worked on the speedo in my 54, years ago, I pulled it out from under the dash.

Anyhow, remove the speedo head from the instrument panel. Spin the round thing where the cable goes. Can you spin it with your fingers? If not lube it up. Spray WD40 into the hole and elsewhere, including the weephol. Also take a Q-tip, put it in your drill, put the drill in reverse, and insert the Q-tip into the hole where the cable goes. This will spin the needle and make your needle move. I have been able to maintain a speed of about 55 mph with my drill.

If your drill spins freely before your lube it up, then your problem is the cable or the gears. If your speedo was tight or frozen then that was your problem and probably what chewed up your teeth.

Pic of my panel coming out the hole in dash.

 

Thanks, Abe. I'm able to get the speedo needle to move when I spin the back with my fingers. So, while I don't know about how realistically it's going to tell me how fast I'm going, it should at least register something.When I got the truck in the early seventies, I bought a new speedo cable for it. I recall that it twisted up and got ugly. Years later I attempted to get the speedo working, and though the cable didn't get ugly and twisted up again, the speedo still didn't work. I think know that the drive gear probably lost its teeth at the same time the cable knotted itself up. It was only today that I noticed the ground down teeth. I've driven this truck across the country three times, but I don't have one mile to show for it on the odometer...

 

At least on the BonusBuilts, you can't just change the driven gear, outside a small range. The drive gear needs to match. Note that they are metal gears, but I still think the wrong combination will chew up a plastic gear.

 

The 48-56 chassis parts manual has a table with all the combinations (page 772 of the big pdf).

As Ross stated there are different part number drive gears that need to match with the driven gear. The most probable is an 8 tooth drive gear. I recommend jacking up the rear end of your truck, pulling out the speedometer cable from the transmission, marking the drive gear, turning the drive shaft, and counting the teeth of the drive gear. This may be a pain in the but requiring flash lights and mirrors, but it will determine exactly which driven gear you need.

If you have a 3.92 axle and your transmission has an 8 tooth drive gear, then you need a 23 tooth driven gear; part number 01Y 17271-F.

If you have a 4.27 ratio axle and your transmission has a 7 tooth drive gear, then you need a 19 tooth driven gear;part number 01Y 17271-B.

If you have any other combination that those two, please post what you find so we can help figure it out with you.

And sine it is late for me, someone please double check my parts book reading...

 

38coupe: I'll count the teeth in the transmission drive gear. Since it is a fair guess that my rear end is the basic that came with the F100s for "53, I'll assume the 3.92 ratio. I'll measure up my tires and see how all this works with the math that papacrunch posted, I should have a pretty good idea of how many teeth I need on the speedometer drive gear.

 

Just remember you probably can't go by the manual due to the tires you are running. Unless of course you are bone stock. But then again did it have 16" or 15" rims from the factory.

 

Either rear axle ratio is common. I'd make sure which one you have.

Since the driven gear needs to match the drive gear I'd go by the manual. If your speedometer is off a lot you can use one of the tiny correction transmissions available for speedometer cables.

 

Pic: Page 747 Scroll down to DRIVE & DRIVEN Speedometer Gears for 1948/56 with 3 speed manual light duty except 1953/56 F100 with 3 speed overdrive (listed on the next page).

DRIVE gear: Located on the transmission output shaft / DRIVEN gear: Located at the end of the speedometer cable where it fits into the trans.

Besides the rear axle ratio, one has to know the original tire size. The rear axle ratio is stamped on the Rating Plate, located on the inside of the glove box door.

Note: The misc 01Y-17271 DRIVEN gears are iron and are obsolete. The misc B5A-17322 driven gears are nylon, Carpenter has them.

 

So, everything points to a 20 tooth speedometer cable drive gear. As I posted earlier, the damage to the teeth probably happened many,many years ago when the cable got twisted up and broke. This was probably due to wrong cable cable lubricant or a cable routing with too tight turns or just something done wrong by me half a lifetime ago when I was young and stupid. Now, I'm old and stupid and trying to get the f***ing speedometer working for the first time in almost 45 years....

 

Nah, the cables just wear out and fray when the lube dries up. Not many people think to pull it out and smear grease on it every couple years. Tight bends will exacerbate the issue. To be honest, mine routes almost straight forward from the speedo, follows along the frame rail next to the master cylinder before making a large, soft bend into the trans. I'll try to get a pic this morning.