Speedo Correction and Math Fun with Eric Zappe and Mark Tener - Page 2 - JeepForum.com
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post #16 of 18 Old 03-05-2013, 08:48 PM Thread Starter
Kevin108
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2000 XJ Cherokee 
 
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Funny thing about stock XJs... When I bought my XJ, it was going stay stock too. It's an admirable goal but rarely does it succeed.

That's awesome on the 97! He's definitely lucky to have someone as knowledgeable as you to help him along with it. Your spare parts collection probably doesn't hurt either.


My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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post #17 of 18 Old 03-05-2013, 09:59 PM
mschi772
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Quote:
Originally Posted by Kevin108 View Post
34.018823569440493607884879472531

34 teeth and epic decimal places!
http://en.wikipedia.org/wiki/Significant_figures

Quote:
Originally Posted by habbyguy View Post
I did it a little different, starting with the premise that I didn't assume my speedo was perfect to start with. Just applying a formula won't fix a speedo error (and US vehicle speedos are often WAY off out of the factory - I think they can be something like 5% off legally).

Anyway, after converting to 31" tires, I went out and did a speedo check run, recording the actual speed (using a GPS) at 10mph increments from 30 to 80 (oops, I must have meant 60, since those last two readings would have required speeding).

Then I counted the teeth on my current speedo drive gear.

Then I figured the ratio between my actual speed and indicated speed for each of the readings I took, and used the average.

Then I divided the counted number of teeth on my current gear by the average ratio, and rounded up or down to the nearest whole number.

It came out really close to a whole number (27 teeth, if I recall correctly). My problem was that the gear was only available in a long-shaft version (for the early 90's cable-drive speedo), so I had to order one of those and melt / glue the 27 tooth gear onto my short shaft.

The results were great - my speedo is off no more than about 1mph at any speed (kinda nice when I pass those "Your Speed Is" signs, they agree with my dash).
This is probably the best way to be accurate because of how INaccurate most stock calibrations are.

Build Thread -- http://www.jeepforum.com/forum/f177/whats-chrokeese-build-thread-2525690/
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post #18 of 18 Old 03-06-2013, 05:33 AM Thread Starter
Kevin108
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I know about significant figures, I just left them there to be silly. The pictures of girls weren't part of the arithmetic functions either.

The first error most people use is by going with the tire size and assuming a 31x10.50-15 is really 31" tall. The second is by just using some chart they found online.

My build thread has pics again. http://www.jeepforum.com/forum/f177/kevin108s-2000-xj-build-867428/
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