Axle Conundrum..

[gogmorgo]

Here's a few:

 

I've found when searching on here, the best way is to use google. Including "site:comancheclub.com" will restrict the results to those found in this domain. 

 

Other good things to look for is technical info about the SCCA circuit racing trucks built by the Archer Bros back in the day. MJs cleaned up in motorsport. It may not be the most advanced tech, but it worked. 

[Dzimm]

4 minutes ago, gogmorgo said:

Here's a few:

 

I've found when searching on here, the best way is to use google. Including "site:comancheclub.com" will restrict the results to those found in this domain. 

 

Other good things to look for is technical info about the SCCA circuit racing trucks built by the Archer Bros back in the day. MJs cleaned up in motorsport. It may not be the most advanced tech, but it worked. 

Thanks.  I'll have to try the Google tip.  

[Eagle]

Cooper Tire still publishes revolutions per mile in their specs. For anyone wishing to accurately figure out RPMs relative to speed, that's the number to use.

 

https://tiresize.com/tires/Cooper/Discoverer-AT3.htm

 

For some sizes, they list three different tires, I guess based on load rating (or speed rating), and the numbers are different. I'm going to have to update my spreadsheets. I'm not going to list three of the same size, because if you look at another manufacturer's tires of the same size, their numbers will be different. A difference of 5 or 10 or even 25 RPM doesn't matter, so where they list multiple entries for the same nominal tire size ... I averaged them.

 

Now I have to get a good night's sleep and come back to triple check my work.

 

And then I have to try to remember how to update the document on Google Docs

[Eagle]

I think I got the updated version uploaded to Google. If the top says copyright through 2018 ... it's the new version.

 

It's as correct as I can make it from the limited real-world revolutions per mile I could find. When I first made up the spreadsheet, almost all the tire manufacturers gave that data. Now almost none do. For tire sizes not listed, I did some comparisons. I spot checked a number of entries in another spreadsheet I have, of tire specs. For ten entries for which I had actual revolutions per mile, I compared those to the revs/mile calculated based on nominal tire size. The difference ranged from 2.64% to3.53%. So, to get in the ballpark, if all you have is the calculated revolutions per mile ... just add 3 percent and you should be close enough for most purposes.

 

I'm not sure if you lads can just view the spreadsheet, or if you can edit it. If you find that you can edit it -- PLEASE DON'T! If you want to play, download your own copy, don't change the one I uploaded.

 

And, please respect my copyright. A fair amount of work went into that spreadsheet. It says it's copyrighted because I am not releasing it into the public domain. Please don't pass it on to anyone else.

[Dzimm]

34 minutes ago, Eagle said:

I think I got the updated version uploaded to Google. If the top says copyright through 2018 ... it's the new version.

 

It's as correct as I can make it from the limited real-world revolutions per mile I could find. When I first made up the spreadsheet, almost all the tire manufacturers gave that data. Now almost none do. For tire sizes not listed, I did some comparisons. I spot checked a number of entries in another spreadsheet I have, of tire specs. For ten entries for which I had actual revolutions per mile, I compared those to the revs/mile calculated based on nominal tire size. The difference ranged from 2.64% to3.53%. So, to get in the ballpark, if all you have is the calculated revolutions per mile ... just add 3 percent and you should be close enough for most purposes.

 

I'm not sure if you lads can just view the spreadsheet, or if you can edit it. If you find that you can edit it -- PLEASE DON'T! If you want to play, download your own copy, don't change the one I uploaded.

 

And, please respect my copyright. A fair amount of work went into that spreadsheet. It says it's copyrighted because I am not releasing it into the public domain. Please don't pass it on to anyone else.

Thanks for the help Eagle, this will help greatly.  I double checked and it is view only so it can't be edited by us. 

[87MJTIM]

Here's a thought - has the OP changed out his speedo gear?

 

Since 235/75/15 was not a stock tire with the MJ, if he still has an original gear installed, he may be reading a lower MPH than he is actually running (ask me how I know this).  So, if his speedo shows 70, he could be +5 MPH higher.  Faster speed = higher RPM

 

My 2¢.

[Dzimm]

3 hours ago, 87MJTIM said:

Here's a thought - has the OP changed out his speedo gear?

 

Since 235/75/15 was not a stock tire with the MJ, if he still has an original gear installed, he may be reading a lower MPH than he is actually running (ask me how I know this).  So, if his speedo shows 70, he could be +5 MPH higher.  Faster speed = higher RPM

 

My 2¢.

The truck is still as it was when I bought it so I highly doubt it was changed.  I'll have to find a speed trap to test it out.  

[87MJTIM]

Mine came stock with 225/75/15.  When I upgraded to 235s, my speedo was off 3+/- mph.  If yours came from Jeep with 215s (stock size on some models), then your MPH may be off 5+/-.

 

http://www.novak-adapt.com/catalog/transfer-case-parts/np-speedometer-gears

 

edit:

if you changed your tire size but didn’t change the speedo gear accordingly, your MPH will be off

[Eagle]

I'm still thoroughly confused because Dzimm refers to the tires by what he thinks the diameter is rather than what the actual size is. He did mention a 225/70 (15?), but (a) I don't know which truck those are on or if those are the tires he used to get his speed and RPM readings for, and (b) that wasn't a size that was offered by the factory, so the speedoemter can't be accurate.

[bad_idea]

The easiest way to verify your speedo is to pull up the GPS on your phone while driving.  Keep in mind your speedo will be off by a percentage, not a constant number.  As you go faster it will be off by a higher number.

[Dzimm]

3 hours ago, Eagle said:

I'm still thoroughly confused because Dzimm refers to the tires by what he thinks the diameter is rather than what the actual size is. He did mention a 225/70 (15?), but (a) I don't know which truck those are on or if those are the tires he used to get his speed and RPM readings for, and (b) that wasn't a size that was offered by the factory, so the speedoemter can't be accurate.

Yes thats the size on the red truck and the ones I've got my actual numbers from (2300ish RPM at 70mph in 5th).  The tires are what came on the truck when I bought it and I highly doubt the po fixed any speedo inaccuracy.  On my way home from work I will use one of those GPS phone speedos bad_idea mentioned to see how far off it is.