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post #1 of (permalink) Old 08-26-2000, 11:19 PM Thread Starter
 
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HELP with math-speedo question

Over the years I've collected several of those little speedometer gearboxes-little gearbox that attatches to the transfer case, and the speedo cable screws onto it-and it speeds up or slows down your speedo reading. So I take a couple apart and there are three gears-one driven from the transfer, an idler, and the one that drives the speedo. These have different tooth count combinations.

My question is does anyone know of a formula to use to figure out how much difference a certain combination of gear tooth count will affect the speedo reading?

I'm not the brightest bulb on the tree, but I did grad you ate kollege, and still couldn't figure out a formula. Trial and error got my 83 CJ-7's speedo dead-on, but I'd still like a formula.

Thanks

Greg 89 XJ-stock, 83 CJ-7/4.0HO-t18-d300-locked-2.5" rancho-32 bfg mt, 79 Blazer/454-t400-np205-locked-3/4 ton axles-35 bfg, 67 CJ-5/231-t18-d18-4.88-front disc-rear 11" drum/front locked/30-9.50's
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post #2 of (permalink) Old 08-27-2000, 04:32 PM
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Re: HELP with math-speedo question

[img]/wwwthreads_images/icons/smile.gif[/img] RPM of driven gear times number of teeth gives you Teeth-per-RPM. Divide that by the teeth on the idler gear and you get RPM of the idler. If you divide the Teeth-Per-RPM of the input gear by the number of teeth on the output gear, you will get output RPM. Then, RPM-of-Input divided by RPM-Of-Output gives the difference you can expect. Remember, the idler gear is MEANINGLESS since it can only relay the number of teeth per RPM from the input gear to the output gear. The only reason it is in tere is so the gearbox doesn't change the rotation of the speedo cable. So, Teeth X RPM=TEETH/RPM (input) TEETH/RPM divided by TEETH = RPM of output. RPM(input) divided by RPM(output) =multiplier for the speed correction.[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
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post #3 of (permalink) Old 08-27-2000, 06:01 PM Thread Starter
 
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Re: thanks!

That makes perfect sense. After I got it right by trial/error, I threw the leftover parts in a plastic baggie, blessed it with a few key words and stuffed it in my goodies box(I'll bet most Jeepers have a similar box in the garage). New tires for the CJ-5 are in the future soon, so this will really help when I go to correct the speedo, no more trial/error. Thanks!

Greg
89 XJ-stock, 83 CJ-7/4.0HO-t18-d300-locked-2.5" rancho-32 bfg mt, 79 Blazer/454-t400-np205-locked-4w disc-35 bfg mt, 67 CJ-5/231-t18-d18-4.88-front disc-rear 11" drum/front locked/30-9.50's
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post #4 of (permalink) Old 08-27-2000, 06:09 PM
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Re: HELP with math-speedo question

CJDave got it right, but you can factor out RPM out of the equation and just divide the first tooth count by the third tooth count, as an example: first gear 15 tooth, second X tooth, third 17 tooth. dividing 15/17=.882, move the decimal place two places to the right yields 88%. the box can also be inverted: 17/15=1.13 or 113%.
I like to use percent because it factors out a lot of math, if you change actual tire diameter X% then also change speedometer gearing X%

good luck, jjc

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