I believe I saw a table at one time to show what speedo gear/color needed for larger tires, but I cant seem to find it now. I'm not sure what tooth count I need here, t19 transmission with 3.55 gear ratio. I moved up to a larger 315 75 r16 tires from factory 235 85 r16 and need the new speedo gear. thanks guys!

 

The formula for caluculating the driven gear number of teeth on the speedo cable is as such



# DRIVE TEETH x AXLE RATIO x TIRE REV. PER MI.
_____________________________________________ = # DRIVEN TEETH

1001


To calculate tire revs per mile


20168
_______________________________ = REVIOLUTIONS PER MILE

TIRE DIAMETER (in inches)

 

thanks!

its about a 35" tall tire, so that puts the revs per mile at 576.

I don't know the drive gear so I cant calculate driven gear though.

 

Time to apply some algebra to find the driven gear..

You know your gear ratio and the size of the existing tires.

So lets solve for X the driven gear.

To do this.


Driven gear X 1001
________________________ = number of drive teeth.

Gear ratio x revs per mile.

You are going to have to pull the speedo cable at the trans to find the number of teeth on the driven gear

 

kinda confused here....
I don't know the drive teeth or the driven gear, how do I solve for this?

?x1001
------------ =?
3.55x576 (2044.8)

 

You solve for this by crawling under the truck and pulling the speedo driven gear to see how many teeth are on it, thats how you solve it. As was already stated.

How do you expect to know how many teeth you are going to need on the new gear with out knowing what you have now ????? You can't magically pull this out of thin air .

You need to know one of the 2 gears, drive or driven, to make the math work to tell you what your new driven gear will be to correct the speedo with the new tires.

 

Gotcha, that's what I was thinking thanks! I'm having a heck of a day with wrapping my head around this and my OTHER problem you are helping with....and I can't feel my fingers anymore because of the cold!

 

Ya these formulas can be frustrating. But you need to have 2 of the values to solve the third in these triangle or triad type formulas. Missing a value in the base formula to solve for X, makes solving X impossible.

 

 

Let me correct myself, 8 would be my new "drive" gear.

 

Redo your calculations again, and use the diameter of the factory supplied tires (30"?) to figure out what the drive gear is. Then, once you have the correct drive gear tooth count, go back and do the calculation again with the tire size you want to change to (35"?). You want to change the driven gear if possible, not the drive gear.

According to my quick calcs here, you'd probably have a 7 tooth drive gear and will want to switch from a 16 tooth driven gear to a 14 or a 15 tooth gear...IF it will work with the installed drive gear. Matthew can correct me if I'm wrong here, but I believe you can only change the driven gear so much before having to change the drive gear.

I just did some reading on this recently since I'm doing a transmission swap soon, but my memory may be a bit fuzzy on the topic.

 

Nope the drive gear stays the same.

You were just trying to find the number of teeth on the drive gear using the existing tires (NOT THE NEW ONES) and the current driven gear with the last formula . You came up with 7.88 that would get rounded up if you were looking for that. But likely you have a 7 tooth driven gear. The drive gear is biatch to replace and 99.95 of the times is not touched.

Driven teeth X 1001
________________________ = number of drive teeth.

Gear ratio x revs per mile (of current tires).



If you used the new tires for the last formula you need to do it again with the current tires.
As you would found what you need for drive teeth with the new tires and that gear is a biatch to replace and won;t be as accurate as changing the numerically higher toothed driven gear.



Once have the number of driven teeth with the current tires, use the first formula to find the new driven gear with the new tires.


# DRIVE TEETH x AXLE RATIO x TIRE REV. PER MI. (New Tires)
_____________________________________________ = # DRIVEN TEETH

1001




Sorry if I did not make it clear, some times I forget and just assume that the mechanics behind this stuff is fully understood and forget that not every one does.

My apologies for the over sight.

 

Nailed it

 

Wow, it's been a while since I've done math.
Going with the original tires, 235 85 r16 at a 31.5" tire, my calculations show a drive gear of 7.04. Plugging that into the original formula using the new tire revolution by mile shows a driven gear of 14.38. Either a 14 or 15 driven gear would be close to the money then.

Sorry for all all the confusion and headaches I caused! Like I said, I'm having a hard time remembering algebra from so long ago, I definitely wasn't good in math!

 

7.04 is right, a near whole number is where it should land.

The issue you have now is they do not make a 14 tooth driven gear that I'm aware of.

This would mean swapping to an 8 tooth drive gear on the trans. This will mean you need a driven gear of 16.35 (16) teeth driven gear.

Driven gears come in 16-21 teeth.
Drive gears come in 6-8 teeth.