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Old 03-15-2015, 12:38 PM
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Speedometer

2004 X, went from stock tires to 31x10.5. Feels much lower gear, but not too bad. I guess the Speedometer is now reading low. Can anyone guess roughly how much…I don't want a cop to tell me!


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Old 03-15-2015, 12:41 PM   #2
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When I went from the "stock tires" I went to BFG 31"s...and when checked with a GPS the speedometer was dead on......so yeah slightly off from factory...of course that would also show higher mpgs.....I had 3.73"s and 5speed. IDK if it applies to all TJs...would depend on what your "stock" size tire was, and what gears?

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Old 03-15-2015, 01:34 PM   #3
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use a gps app on your phone to check. I did the same thing and at 70 i'm actually going 75.
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Old 03-15-2015, 02:02 PM   #4
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Agree chech with GPS

Rough estimate is based on tire diameter since circumference is porpotional to diameter

So if you increase tire diameter by X% speedo reads low by sane X%

For example if your speedo was dead on with 30 inch tires and you switched tires to 33 with no change in diffy gears
So 30 to 33 is a 10 percent increase so speedo now reads 10 percentl lower at any speed
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Old 03-15-2015, 03:32 PM
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Good ideas, thanks.

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Old 03-15-2015, 03:35 PM   #6
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Before I replaced the spedo gear I was about 5 mph off with my 31s at highway speeds
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Old 03-15-2015, 06:29 PM
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I presume that was 5 mph slow? That would match my expectations.

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Old 03-15-2015, 09:01 PM   #8
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I have the same question but I'm about to run 32s. There not an aggressive tread so should it be off by 8-10 mph?
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Old 03-16-2015, 09:02 AM   #9
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When I was running 32's I checked the speedo with a GPS and couldn't tell the difference.
Results will vary with different tire manufacturers having slightly different sizes.
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Old 03-17-2015, 02:53 AM   #10
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I'm running new 31's and I am almost 7mph 'slower' on my speedo going 70ish. My garmin glo will read 80mph and my speedo is 72ish mph.

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