Someone wanna count their teeth?

[jdconnel]

Wondering what the proper speedometer gear teeth count should be for a 3 SP NP-435 transmission. Truck has a Dyna 60 rear end with a 4.10 G.R

I know I asked this once, and was told to “order a few teeth lessâ€Â

[averagef250]

Tire size is also a huge factor in choosing the speedo gear.

[FORDification]

The following link should be helpful to you:

http://www.tciauto.com/Products/TechInf ... _gears.asp

[jdconnel]

Ok, here is goes…..

I have the original rims (16.5's). I think I have 8.75LT16.5 tires on the truck... going by memory. To, transmission, tires, and rear end are all factor original specs.
I found similar tires listed as 225/50R16.5 (gives me my .5 aspect ratio, unless you have a better number)

Section Width: 8.75 in
Aspect Radio: 0.5
Rim Diameter: 6.5 in
Tire Diameter: 25.25 in [2 * (8.75*0.5) + 16.5]
*Note: number feels right from when I’ve had a tire off….
Tire Circumference 79.3 in
Tire Rev. per Mile: 798.7

# Drive Teeth: 28
Drive gear refers to the gear that is on the transmission output shaft. Based on the transmission section "28-spline output shaft"

Axle Ratio: 4.10

# Driven Teeth = 91.6 (based on the math):

(# Drive Teeth X Axle Ratio X Tire Rev. per Mile) / 1001 = # Driven Teeth

“The driven gear is located in a removable housing usually in the tail housing of the transmissionâ€Â

[spartman]

I think that for the tires you have the aspect ratio should be different.

I know that the older style tire sizing had more sidewall to tread, whereas the 50 series P-metrics have more tread to sidewall thereby affecting your aspect ratio.

If I were you I would use a tire that has a sidewall ratio of either 75 or 80/85 from a P-metric tire.

[19674x4]

3-spd np-435, are you sure. it's a 4speed.

[jdconnel]

Sorry, typo. 4-speed
Just never use 1st

[kaptnkaos]

Hey JDC...

I just counted mine ... none ....




KaptnKAOS

[jdconnel]

Re-ran my above calculations with # Drive teeth = 7, kept my ratio at .5, and the answer is 22.9.

I am basing the “7â€Â

[MadMaxetc]

I think you are making it more complicated than it needs to be.

Since your RPM, Tire Size, and Gear Ratios are all the same it is just a matter of simple Ratios.

You say that at 60mph (tailing a buddy going 60?) you are reading +15mph, so 75? If I am understanding this right then...

with an 18T gear, 75mph/60mph = 1.25.
1.25*18 = 22.5T (ideal)

A 22T gear will make you read a little fast.
A 23T gear will make you read a little slow.

The only problem is that they only make a 21T at moset.

What gear do you have on the cable? You may have to change that too.

Post that and I will re figure.

[jdconnel]

There's a gear on the cable too? Like behind the dash?
Don't know what that one is.

If you look at that link I posted though, it mentions a 23T gear for use with 4.10 ratio... which is what I have.

I'm going to run a GPS and drive to get my speed to speed conversions figured out for sure. Should be linear.

And yeas, buddy tailing me at 45mph. dash read 60.

[MadMaxetc]

There is one in the Trans (18T to 21T I think) and there is one on the end of the cable that plugs into the Trans (16T to 19T I think).