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Old 05-31-2011, 01:53 PM   #1
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Speedometer Gear Calculations?

How many of you guys have used to trip meter/odometer vs. milemarkers or gps distance to calculate your speedometer error? Is this a known accurate method for the TJ? I recall someone saying that this may not be accurate but I would think that it works fine.

My other quandary is that by my calculations, a 31 tooth speedometer gear would be better than a 32 tooth speedometer gear for my TJ (AX 15, 3.55 gears, 31" KM2s). Every single chart I see indicates that a 32 tooth would be right for me. The one assumption that I'm making is that my Jeep currently has a 33 tooth gear since it came with the Sahara package which had 30" tires standard...I could be wrong and will pull the gear tonight to check.

Anyway, I drove a 50 mile stretch of highway between San Antonio and Corpus Christi and my trip meter read 47.4 miles when it should have been 50 miles on the dot; that's just over 5% error and theoretically a 31.284 tooth gear would be perfect (we all know that's impossible). It would seem like a 31 gear would work great, with less then 1% error and an indicated reading of 70.6 mph while doing 70 mph.

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Old 05-31-2011, 08:47 PM   #2
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Not much of a difference with my 31 inch tire. Considering actual dia. Is only 30.5
Chart calls for a 27 tooth and Mopar does not even make one for my application

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Old 05-31-2011, 09:08 PM   #3
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My current setup is just inaccurate enough to annoy me. I just pulled my gear and I was right about it being a 33 tooth. Now I guess I'm looking for a 31 tooth unless anyone sees a flaw in my calculations!
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Old 06-05-2011, 01:15 AM   #4
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The charts for my jeep showed I needed a 41 tooth gear.
Thats 5.13 gears and 35x12.5x15 tires, however, it was still off according to a gps.
My tires don't measure 35" tall, more like real close to 34" so I put in a 40 tooth gear and did another gps trip, its almost perfect now.
Now I have a spare 41 tooth gear for my next tire swap.
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Old 06-17-2011, 11:05 PM   #5
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I just installed my 31 tooth gear and my calculations were correct! At 70 mph on gps, I now have in indicated 71 mph on the speedo. I also did a 10 mile run and it was less than 1/10th of a mile ahead in error. Big difference in accuracy over my old gear! Now I can actually run some decent fuel mileage calcs....

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