Hi all. I recently bought a 1981 F-150 4X4 with a 4.9 and a 4 speed. The speedo isn't working. I pulled the cable, and there was no pinion gear on the end. I'm guessing the retaining clip disengaged and the pinion gear is laying somewhere in the bottom of the tranny case right now.

The cable appears in good shape, and when I hook a drill to it, the speedo seems to work just fine. So that missing gear is the problem. I called my local Ford dealer, and they said they had no idea what gear might have come with the truck (everything on this truck is, I believe, stock), and all their possible parts are obsolete and unavailable anyway. I called PATC (a company that specializes in just those sorts of parts), and they told me it could be any of several gears, and I would have to provide them with the number of teeth or the color of the gear for them to send me a replacement.

The truck has 31 inch tires, and I have no idea what the final drive ratio is.
Does anybody know.... was there a standard stock speedo pinion gear that came with this beasty? How many teeth or what color? If I can get that info, they can sell me the parts I need, without having to buy several that I will need to install and clock for speed accuracy.

 

No one will know until you tell them the rear axle ratio. And, when you say "31 in tires" is that the rolling diameter or height? We have to get it back to a tire size that was used in the 80's as that's what the tables have.

I'll be specific: If you give me the rear axle ratio, the transmission (though I'm not sure I need it), and the tire's rolling diameter I can find the gear you need. And, you can get the rolling diameter from Tire Rack.

 

Assuming the tires are exactly 31 inches in diameter, you can find the circumference by multiplying the diameter * Pi (~3.14...) which comes to.. about 97 and 1/3rd inches rolling diameter. A tire with a 31 inch rolling diameter would be... somewhere in the neighborhood of about 10 inches tall.. a bit too small for our trucks (even if it is an F150 ) Sorry Gary, I just had to mess with you.

 

Yep, I recognize "messing" when I see it. And, btw, it really is 3.14159 ..... and then I lose it. Anyway, I'm sure you recognize I'm not wanting to do all the work. Tire Rack lists a bunch of 31 inch tires and their diameters vary all over the place. While that probably won't matter as there aren't that many different gears, let's get it as close as we can. So, I'm hoping ULA will come back with some specifics so I can answer the question w/o doing all the work.

 

A couple thing you will need to know to figure it out.

1)Drive Gear Teeth (on the inside of the tran) sometimes you can shine a light in there and count as you turn the output shaft, or maybe someone on here would know.

2)Rear axle ratio

3)Tire revolutions per mile


Here's the equation:


Drive Gear teeth x Axle Ratio x Tire rev. per mile Divided by 1000 = tooth count you will need


So, if you have 7 drive teeth with 3.73 gears and a 225/60R15 tire (which would be 815 revs per mile divided by 1000 = 21 tooth gear.


Hope this helps.

 

Or, use the table in the master parts catalog that already knows about the gear in the tranny and lists the possible axle ratios vs the possible tire sizes. Either way you absolutely have to know the axle ratio and the tire diameter.

 

Thanks Gary.

The original tires (according to the door plate) on the truck were L78-15c. Nothing comes up in the Tire Rack search when I put that in. The tires currently on it are Michelin 31 10.50 -15. The Tire Rack shows the actual diameter for those tires as 30.8 inches. The wheels on the truck now are aftermarket, so these Michelins were fitted to them.

The transmission is a New Process 435 (Doorplate Code "A"). Doorplate Axel Code= 16. I've looked that up on two different web sites and found it listed as a 3.73. But my Chilton's manual says it's a 3.50. I suspect the 3.73 is correct, but will confirm that by jacking up the rear end and counting wheel/drive shaft revolutions. Can't do it at the moment, because I don't have a garage bay available, and the truck is outside, and it's raining cats and dogs. Hopefully, you have a reference you trust.

Someone told me I needed to get you the transmission tag number for you to know whether the speedo drive gear was 6, 7 , or 8 tooth. I'll have to locate that. Wherever it is, it's buried under about a 1/4 inch of old grease.

I really appreciate the help!

 

Axle code 16 is a 3.50 ratio. That, the tranny, and tire diameter gives me all I need. Will check it out in a bit and let you know, but no need to count rev's.

 

Attached are three pages from the Master Parts Catalog. From the first one you can see that with a 3.50 axle ratio and P235/75-15 tires and a 4-speed tranny you need speedo gear C1DD 17271-A, and in the 2nd page you can see that gear is a Type 2A with 20 teeth.

BUT, you are running bigger tires than the 235's, which Tire Rack shows to be 28.9" in OD, while it shows your tires as 30.9" OD. So, your axle will be turning slower and your speedo will be slow with the 20-toothed gear. In order to make the speedo turn faster you need a smaller gear. Dividing the OD of the 235 tires by the OD of your tires (28.9/30.9) gives .935. Multiplying that times 20 teeth gives 18.7 teeth. However, the gears only come in an even # of teeth for some odd, or even, reason and the nearest you'll get is a 19-toothed gear.

Going back to the 2nd page you'll see that C0DD 17271-C is a Type 2A 19-toothed gear. Viola!

And, just for grins the last page shows that the NP435 has an 8-toothed drive gear, so you can use that to run 81CP's equation if you want.

 

I am in awe! Thanks so much!

 

Glad to help.

 

He said he has a 4x4. That means the speedo gear is in the transfer case. Does that make a difference in all the above calculations?

 

Maybe. I'll take a look - later. But, thanks for catching that.

 

Actually, it doesn't change anything that he has a 4x4 because the t-case used the same tooth-count on the drive gear as the transmission. I've included the same page from above to make it easy to see.

(Once upon a time I was wrong, and that was when I thought I was wrong.)

 

FYI, I confirmed your recommendation using 81CT's formula for calculating the number of teeth needed in the driven gear. It came out to 18.93.....i.e., the same 19 tooth gear you recommended....... so GOOD CALL! And thanks again.