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Old 11-02-2010, 10:29 PM   #1
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Speedometer

ive had my 31s for a couple months and i was wondering if they have an affect on the accuracy of the speedometer. theyre alot bigger than the stock tires so i figured they might.

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Old 11-03-2010, 12:02 AM   #2
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Old 11-03-2010, 07:21 AM   #3
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Run with a GPS.
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Old 11-03-2010, 07:29 AM   #4
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Quote:
Originally Posted by Red Wrangler View Post
i was wondering if they have an affect on the accuracy of the speedometer.
Yes. not much but a few MPH's.

Look on your rear diff cover and find the gear ratio ... will be stamped either 3.07 or 3.73. here....





Then look at this chart and find the speedo gear you need for your tire size and gear ratio....




And then if you deem you want to change it ...order a new speedo gear and change it.

how to change it.....takes 2minutes....
Changing Your Jeep Speedometer Gear
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Old 11-03-2010, 07:45 AM   #5
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I had a cop tell me how much I was off and it was around 3 4 mph with 31s
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Old 11-03-2010, 08:29 AM   #6
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the other half of the coin?

Yes going to a bigger tire will change the speedo. I just went from 31" to 33" and of course it changed. My question is how about the mileage? Is the odom still correct? When the odom clicks the mile...did I go a mile? I don't have a gps to check or I would. If this is off is this something I can fix or do I have to take it or a dealership?
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Old 11-03-2010, 09:06 AM   #7
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Yes going to a bigger tire will change the speedo. I just went from 31" to 33" and of course it changed. My question is how about the mileage? Is the odom still correct? When the odom clicks the mile...did I go a mile? I don't have a gps to check or I would. If this is off is this something I can fix or do I have to take it or a dealership?
Yes your odo is off too. Do the speedo gear swap mentioned above and all will be right with the world again. As it is now, you are travelling farther than your odo reads.
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Old 11-03-2010, 09:09 AM   #8
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I ignore the speedo and odometer. Just run behind the traffic. Police cannot pull everyone over and will pick on the fastest. It's like fishing in a school of fish, you cannot get all of them w/o a net.
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Old 11-04-2010, 06:33 PM   #9
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whats the stock tire size that comes on the 98 TJ?
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Old 11-04-2010, 07:19 PM   #10
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Look on the driver door jam. It'll tell you. Or run your vin through jeep.com they'll send you a build sheet.
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Old 11-04-2010, 07:26 PM   #11
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I just went from the stock 28's? To some 31s and checked with a gps, it was maybe a mile or two mph off. Not worth the hassle in my opinion. Might change if I go to 32s though
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Old 11-04-2010, 11:20 PM   #12
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My 1997 SE has 4.10 gears and was built with "pizza cutter" 15x6 wheels and 205/75 tires. I installed 31" MT/Rs without changing the speedo gear. When I checked it via GPS, I learned the speedo now reads slightly slower than my actual speed--when it reads 45 MPH, I am actually doing 47 MPH.

My 1998 Sport has 3.73 gears and was built with the 30" wheel/tire group. I swapped the same 31" MT/Rs from the SE onto this Jeep, again without changing the speedo gear. When I checked it via GPS, I learned the speedometer now reads dead on. Even up to 65 MPH, it is in exact agreement with GPS.

If you have an iPhone, download the free app called Dashboard. This will give you a quick, easy GPS speeometer function whenever needed.

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