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Discussion Starter · #1 ·
I need a new speedo gear and I found them at A&A trans, and I think I need either a 24 or 25 tooth gear. I can't seem to find the link anywhere on this site to another site that had a speedometer gear calculator. There was a post not too long ago about it. So does anybody know what link that is? And also are the speedo gears in the 2wd trannies the same as the 4 wd trannies?
 

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I went to my local dodge dealer & picked 1 up befor.. they had a box that they let me look through.. they even just swapped mine for the right one... Weaver Motors co. is a good dealer to deal with...
 

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Discussion Starter · #4 ·
taz_man440 said:
I went to my local dodge dealer & picked 1 up befor.. they had a box that they let me look through.. they even just swapped mine for the right one... Weaver Motors co. is a good dealer to deal with...
TX is a long ways to drive for a $15 part lol. I wish I could find that link tho so I would know which one to get exactly...I'm still searching tho..

Ok heres what I found.
Duffy had posted this
"Here's the fomula that you requested
first you need to figure the driveshaft rpm at 60
speed x gear ratio x 336 / tire diameter
60 x 4.56 x 336 / 33 = 2786 ( or about )
take that number and devide it by 75
2786 / 75 = 37 tooth gear needed
each tooth on the speedometer gear is equal to about 3 mph.
speedometers cost about $12 at the dealership."

So going by that formula I need a 25 tooth speedo gear.

But one more question to throw out before I go to bed. If in that formula you change the speed to say 70 instead of 60 it says that I need to have 29 tooth speedo gear? Which is right and if I go with the 25 tooth one, will my speedo be way off at 70, or is the difference negligible?
 

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Dodge 85 RC said:
But one more question to throw out before I go to bed. If in that formula you change the speed to say 70 instead of 60 it says that I need to have 29 tooth speedo gear? Which is right and if I go with the 25 tooth one, will my speedo be way off at 70, or is the difference negligible?
If you change 60 to 70 in the formula, then you have to change the constants as well. It should come out the same.

There was another post that said to drive the truck at 65 on the speedo and time the mile posts to find out how fast you are really going. Then count the teeth on your current gear. Multiply that by the ratio of how fast you were really going and how fast the speedo indicated. That should be the right gear. Hopefully it would agree with the formula you have :)
 

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Discussion Starter · #6 ·
But what are the constants and how do I change them to fnd out? Truck is a no go for driving it so, I need to do it the math way to figure it out.
 

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a quick and cheap conversion is using the little gear box that screws onto the outside of the transmission where the cable normally attaches. picked up a couple in a jy and got my r/c within 5mph of true speed.
btw, used to live by diamond lake near where the msra had their garage. Doug
 

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Discussion Starter · #8 ·
Not really sure what you mean doug? Anybody else know about this formula?
 

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the way it seems to make sence in my mind is that to take the stock tire size and figure out its total height. then take you current tire size and figure out its total height. for me the 2nd is smaller (2wd). then just figure the percentage difference and make up for it by compensating that much the corrasponding way.

example for my 2wd rc.
318 w/3.53s
stock tires: 235-75-15 28.9" total (going by addvertised size)
current tires: 255-60-15 27.1" total (going by addvertised size)
to find the height do this: multiply the first number (235) by the second as a decimal (.75 or 75%). then double that. that gives you (352.5mm) convert it to inchs by dividing it by 25.4. then add the inches of the rim (15") thats how you get the total height.
after all that figure how much bigger/smaller the 2nd is than the 1st. mine is 93.7% of the stock ones. that means i need a gear 93.7% the size of stock (or as close as i can get) once i count how many i have stock i can see what i need.

note: this only figures the changes for different tires...gears i guess would be the same.

sounds complicated but its pretty simple once you've done it. later, and hope this helps somebody.
 
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