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Unread 02-21-2013, 09:31 PM   #1
Kevin108
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Speedo Correction and Math Fun with Eric Zappe and Mark Tener

Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide."

Formula 1:

Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count


Formula 2:

(63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear


My algebra days are long behind me, so can I get someone to double-check my numbers here?

Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my 235s with a diameter of 28.25".

(63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear
28.25 x 3.14159 = 88.7499175
63360 / 88.7499175 = 713.91615659811740106687986498692
713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036
2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531

34 teeth and epic decimal places!

Now, back to Formula 1...



I didn't change the gear ratio, so old / new would be 1, so I'll leave that part out.

28.25 / 30.625 x 34 = New Tooth Count
28.25 / 30.625 x 34 = 31.363265306122448979591836734694

So I need to go all hillbilly and lose 3 teeth?



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Unread 02-21-2013, 10:01 PM   #2
moses21
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Just use this and email Chrysler for your building sheet. Who needs math
http://www.jeep4x4center.com/jeep-sp...ears/index.htm
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Unread 02-21-2013, 11:06 PM   #3
Kevin108
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I have my build sheet. Even before that, I counted R&P teeth and knew what my gears are. The build sheet doesn't tell you what your speedometer gear is. I think you misunderstand the point here.
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Unread 02-22-2013, 08:04 AM   #4
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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).
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Unread 03-03-2013, 04:12 PM   #5
Kevin108
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Got the speedo gear installed last week. The formulas worked like a charm. Out came the 34-tooth, in went the 31. Speedo is GPS verified and spot-on.
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Unread 03-03-2013, 10:48 PM   #6
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Got the speedo gear installed last week. The formulas worked like a charm. Out came the 34-tooth, in went the 31. Speedo is GPS verified and spot-on.
Kevin, sorry I did not see your post sooner. I'm glad the formulas worked for you. I spent countless hours verifying that they worked and as you know by reading the book, I verified them with my GPS as well. The formulas are more accurate than ANY speedo gear chart I have seen.
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Unread 03-03-2013, 11:46 PM   #7
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Every chart I looked at told me to go with a 32-tooth gear. Besides everything else I've learned, your book saved me at least the $40 I'd have spent buying another gear, had I gone by the charts and not by the formulas. Some of the best money I've spent in my 4 years of XJ ownership, for sure.

Since you've responded (I always forget you're a member here!), can you explain any of the second formula? Mainly, what do the numbers 63360 and 74.5 represent? I've asked some very intelligent people about this and not gotten any real answers.
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Unread 03-04-2013, 12:48 AM   #8
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OP, did you actually measure the diameter of your tire with a tape measure or use the numbers provided by the manufacturer?
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Unread 03-04-2013, 01:39 AM   #9
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Quote:
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Since you've responded (I always forget you're a member here!), can you explain any of the second formula? Mainly, what do the numbers 63360 and 74.5 represent? I've asked some very intelligent people about this and not gotten any real answers.
63360 is the number of inches in a mile. 74.5 is a secret that only Mark Tener knows. Honestly it has been several years since I asked him about the formula and I don't know the answer to that.
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Unread 03-04-2013, 06:49 AM   #10
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Originally Posted by EricsXJ View Post
63360 is the number of inches in a mile. 74.5 is a secret that only Mark Tener knows. Honestly it has been several years since I asked him about the formula and I don't know the answer to that.
I understand. Is he just a buddy of yours or a guy I might stumble across on here or NAXJA? It doesn't really matter, I'm just a nerd.

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OP, did you actually measure the diameter of your tire with a tape measure or use the numbers provided by the manufacturer?
I measured them. I have a work bench in my garage with 4x4 legs. I rolled the tire up against the bench leg then on the opposite side I held a framing square against the floor and slid it up to the tire. I made a mark on the floor where the edge of the square was touching the tire, then measured from the bench leg to my mark. That gave me the diameter of the tire. I did this for both tires.
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Unread 03-04-2013, 09:38 AM   #11
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I understand. Is he just a buddy of yours or a guy I might stumble across on here or NAXJA? It doesn't really matter, I'm just a nerd.
No he had his formulas posted on a jeep newsgroup (what we had before Jeep Forum) and I asked if I could use it in the book. I don't know if the guy is still around or not.
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Unread 03-04-2013, 02:25 PM   #12
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Unfortunately I too am old enough to know what a newsgroup is. Every time I look through your book, I wonder how many of the guys are still in XJs and what their builds have become. You should write a follow-up for your web site!
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Unread 03-05-2013, 12:18 AM   #13
EricsXJ
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Unfortunately I too am old enough to know what a newsgroup is. Every time I look through your book, I wonder how many of the guys are still in XJs and what their builds have become. You should write a follow-up for your web site!
I'm in the process of writing my second book now. Should be ready for purchase in 12 months or less. I'm still friends with many of the guys that had jeeps featured in the first book. I won't post their last names but if you have the book you will know who I am talking about.

Travis got rid of his white jeep. He has since built a highly modified XJ caged and all that but I think he still runs 33's.

I am not sure what Lee is doing. Its been a few years since we talked. I know he took several expeditions with his XJ since the book was put out.

I am still friends with Bryan and he's one of my wheeling buddies. He still has the same jeep only now it sits on 37's and 44's front and rear.

Garrett sold the Green XJ and has since moved on to different things. He is a friend and I still see him around.

I am not sure what Pete is doing or if he still has his MJ.

Jason is still around and I know he still has his Red XJ but I don't think its changed much from what is in the book.

John got rid of his XJ, body had issues I think. He was (is?) still around several different online forums but I don't know if he picked up a new jeep yet.
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Unread 03-05-2013, 06:39 AM   #14
Kevin108
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That's really great news! Glad to see you're still in it and still in touch with the guys.

I'm really looking forward to the new book. Thinking about some of the early pages in the original book, are you building your son his own XJ yet?

Lee's rig is what I hope I'm able to build mine into some day. The wheeling nearby doesn't justify going too much bigger than that although you wouldn't know it from all the overbuilt JKs you see everywhere.

I'm still in the process of convincing my wife this is a safe hobby and something she'll enjoy. If she can get past being scared, I know she'll enjoy it. I've wheeled almost ever since I started driving. She's had her own ATV for 4 years now and tears around on it with reckless abandon but give her a seatbelt, a roof, an airbag and a federal crash rating and she's scared to death of little more than a speed bump. Of course, that was several years ago when it was still stock. Things might be different now...
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Unread 03-05-2013, 08:25 PM   #15
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Yes we picked up a 1997 XJ that needed a lot of work. My son drives it to school every day. We put a 2" budget boost on it but thats about as far as we'll go as far as off-road modifications go. My goal is to make it (or keep it) into a very nice "stock" daily driver.
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