speedometer gear calculations

rkneeshaw · Dec 9, 2009

So I am about to put 32's on my 4x4 ranger and between the non-stock tires that are on there now, the bouncy speedo, and the fact I dont know any of the 185k miles of history on this truck, and the fact that the speedo currently is not very accurate, I'd like to run some fresh calculations to find out what speedo gear I SHOULD have in there and just pretend whatever is in there now doesnt exist or is just plain wrong.

I've got an R6 axle which is a 3.73 limited slip, and the 32" tires I'm installing run 649.7 revs per mile. There's a formula on the tech library that I think might help me out but my algebra skills have been long forgotton:

Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000

Given the info I have, the formula looks like:

Driven Gear Teeth = (Drive Gear Teeth x 3.73 x 649.7) / 1000

Can any of you math wizards help me solve this equation?

Destroyer000 · Dec 9, 2009

16.9 tooth speedo gear.. so a white 17 tooth gear should solve your equation... i believe you have a pink 19 tooth gear.. ive mess with this SOO much its stupid.. iam starting to remember them by color an what tooth they are.. lol

Yeah dont listen to me i SUCK at math.. i think thats wrong haha

rkneeshaw · Dec 17, 2009

I wonder how I would complete this calculation?

LittleHorse · Dec 17, 2009

you've got two variables and only one equation, so it can't be solved. You need to know the number of drive gear teeth, the number of teeth on the gear that drives the speedometer gear. I don't know how to find the number of teeth on that gear, but somebody should know because they ought to be the same for any BW1354 transfer case.

rkneeshaw · Dec 18, 2009

I guess the other way to approach this is if I was able to find out what gear was in there in stock form, then I could calculate off that.... Anyone know what color gear is stock for a 4L 4x4 manual with 3.73 axle?

strvger · Dec 18, 2009

i just check my speed with the local police when they set up their radar speed check display. it says i'm doing 32 and my speedo says 30, i know exactly. i also use the calculator in the library forum and that keeps me close and legal.

LittleHorse · Dec 18, 2009

strvger said:
i just check my speed with the local police when they set up their radar speed check display. it says i'm doing 32 and my speedo says 30, i know exactly. i also use the calculator in the library forum and that keeps me close and legal.

I think that would make me speed even more, with the knowledge that if I get a speeding ticket I can just buy a speedometer gear and show the receipt for fixing it to the judge and get out. :icon_rofl:

Plus I'm just obsessive about doing things the right way.

LittleHorse · Dec 18, 2009

rkneeshaw said:
I guess the other way to approach this is if I was able to find out what gear was in there in stock form, then I could calculate off that.... Anyone know what color gear is stock for a 4L 4x4 manual with 3.73 axle?

That's the way to go.

strvger · Dec 19, 2009

well, i wouldn't know about speeding tickets, i've never had one. 'course i've only been driving for about 46 years.

rkneeshaw · Dec 19, 2009

rkneeshaw said:
I guess the other way to approach this is if I was able to find out what gear was in there in stock form, then I could calculate off that.... Anyone know what color gear is stock for a 4L 4x4 manual with 3.73 axle?

I pulled the gear thats in there now, its Yellow, now I just dont know if that is stock or not....

rkneeshaw · Dec 21, 2009

Having a hard time finding answers, maybe my search skills leave some to be desired..... anyone know how many drive gear teeth are inside a 1354 transfer case?

EDIT: This thread over on explorerforums seems to give a little info, but its confusing. One guy says 8 teeth, but the picture looks more like 6 gear teeth: http://www.explorerforum.com/forums/showthread.php?p=1718380

beaglegun · Mar 8, 2010

I'm struggling with the same problem. I put 33's on and can't figure out what driven gear to buy to get my speedometer and odometer right. I'm guessing I need to count the teeth on the drive gear in order to do the math and figure out what color driven gear to buy. I removed the speedometer sensor from the transfer case but I can't see into that hole, even with a mirror. Surely there is info somewhere that will give me the info on the drive gear?????

rkneeshaw · Aug 16, 2010

I found what I needed:

http://www.grimmjeeper.com/gears.html

You need to know your tire size, rear gear ratio, the transmission you have (MD05-R1), and transfer case (Borg Warner 1354). Plug it in and it will tell you what speed you are going in x gear at x RPMs. Then use that data to plug into the other calculators and get the right speedo gear.

For me, at 2000 RPM in 5th gear, with my 3.73 stock gears and 32" tires I'm doing 65mph. Turns out, my speedo gear is within 2mph or so at that speed, so I'm all set.

v-8power · Oct 30, 2010

Destroyer000 said:
16.9 tooth speedo gear.. so a white 17 tooth gear should solve your equation... i believe you have a pink 19 tooth gear.. ive mess with this SOO much its stupid.. iam starting to remember them by color an what tooth they are.. lol

Yeah dont listen to me i SUCK at math.. i think thats wrong haha

Where can you buy these speedo gears at? Does it have to be a tranny shop? Thanks for any info.

rkneeshaw · Oct 30, 2010

v-8power said:
Where can you buy these speedo gears at? Does it have to be a tranny shop? Thanks for any info.

I found them on ebay for about $5 each.

speedometer gear calculations

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