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Unread 05-28-2007, 08:18 AM   #1
Lupulin
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How does larger tires effect speedometer and odometer?

Hey guys. I've got a a set of 33 x 12.5's on my 81 cj7 jeep. How does this effect my speedometer and odometer readings? Do you have a conversion ratio? I would make me think that the speedometer and the odometer would read incorrectly - to the low side. Does anyone have the size of the factory tires?

81 CJ 7

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Unread 05-28-2007, 09:27 AM   #2
gosupes
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The factory size was somewhere around 28". There are speedometer conversion tables all over that will give you an idea of how fast you are going by using RPM, tire size and gear ratio. The circumference of the larger tire is almost 18% more than the stock tires, so your gauge will read about 18% low.
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Unread 05-28-2007, 09:41 AM   #3
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if you have a tranny with a 1-1 top gear no overdrive I'll be willing to bet your about 5-7 mph under the the actual peed your going..
so if your speedo's showing 45 your really going 50
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Unread 05-28-2007, 10:31 AM   #4
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You can't do a straight assumption of speed. The error gets larger the faster you go, so you need to know the % and do the math. Or get a new speedo gear and call it good.
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Unread 05-28-2007, 01:45 PM   #5
fatterman
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http://www.jeeptech.com/javaAps/Speedo.html

this site will tell you what tooth count you need.
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Unread 05-28-2007, 02:50 PM   #6
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Run your odometer against highway mileage markers. It will give you a good idea of how much you're off.
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