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Old 10-29-2010, 12:02 AM   #1
dow
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speedo gear problem

Hi everyone, i just bought my first jeep and the speedometer is way off. Its an 03 sport with I6, auto tranny, and 33's. The guy i bought it from said it had 4.10 gears (which i assume to be 4.11 for auto), so i ordered the 36 tooth speedo gear. At 65 mph (gps) with the stock 34 tooth, the speedo was reading around 80 mph. With the 36 tooth at 65 mph, the speedo is reading around 75 mph. Rpm's at 65 mph (gps) are around 2250 to 2300. Does this sound like 4.56 (or 4.88) to anyone? It's not driving so well because it shifts into OD too early. If this setup actually is 4.56 and 33, the chart calls for 39 tooth gear, but would using a 40 tooth gear prolong OD from kicking in for a bit? Any help is appreciated!

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Old 10-29-2010, 12:20 AM   #2
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I've got a 99 5 speed and I have 4.56's and 33's and I use the 39 tooth and mine is right on.

Don't know if that helps but figured id chime in.

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Old 10-29-2010, 07:29 AM   #3
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Jack up the rear end, spin your tire one revolution and count how many revolutions the driveshaft makes. One full rev. of the tire should be barely over 4 rev. for the driveshaft for 4.10 gears
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Old 10-29-2010, 08:49 AM   #4
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Originally Posted by essie92 View Post
I've got a 99 5 speed and I have 4.56's and 33's and I use the 39 tooth and mine is right on.

Don't know if that helps but figured id chime in.
I have the exact same thing here and mine is still off by 3 or 4 mph. Reads fast but no big deal.
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Old 10-29-2010, 11:23 AM   #5
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Quote:
Originally Posted by dow View Post
Hi everyone, i just bought my first jeep and the speedometer is way off. Its an 03 sport with I6, auto tranny, and 33's. The guy i bought it from said it had 4.10 gears (which i assume to be 4.11 for auto), so i ordered the 36 tooth speedo gear. At 65 mph (gps) with the stock 34 tooth, the speedo was reading around 80 mph. With the 36 tooth at 65 mph, the speedo is reading around 75 mph. Rpm's at 65 mph (gps) are around 2250 to 2300. Does this sound like 4.56 (or 4.88) to anyone? It's not driving so well because it shifts into OD too early. If this setup actually is 4.56 and 33, the chart calls for 39 tooth gear, but would using a 40 tooth gear prolong OD from kicking in for a bit? Any help is appreciated!
If it does have 4.10, it has both 4.10 and 4.11. The front and rear axle ratios are always off from each other by .01 due to the front and rear ring gear diameters being different from having two different axles (Dana 30 front, Dana 35c or Dana 44 in the rear). Only the Rubicon, because its front and rear axles are both Dana 44 with the same diamter ring gears, have identical front and rear ratios. Same with 3.73 which is a mixture of 3.73 and 3.74, etc.

But to know exactly what speedometer gear to use, if the 36 tooth is producing inaccurate results, you're going to have to verify that it has the 4.10/4.11 ratio.

Based on my calculations and that you say you are getting approx. 2250 rpms at a GPS indicated 65 mph when in Overdrive, you have a 4.88 ratio, not 4.10. 4.10 would produce more like 1872 rpms at 65 mph with a 33" tire diameter when your transmission is in Overdrive. Calculations for 4.88 exactly produce 2250 rpms at 65 mph when the transmission is in Overdrive with 33" tires.
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Old 10-29-2010, 12:35 PM   #6
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Thanks for all the quick replies! Jerry, thanks for the info on the slight ratio difference between front and back. Your probably right about the 4.88 because the 36 tooth only bumped it up a little and I still don't think the 39 tooth recommended for 4.56 would quite get it there. I'll do the tire/driveshaft rotation to see if I can get verification before I drop another 50 bucks (shameful for a piece of plastic on a stick). Hopefully it will be 4.88 because last night I think I saw an old post of yours suggesting that would be the best setup for this tire size and transmission (42re I think).
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Old 10-29-2010, 12:39 PM   #7
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For 33" tires and 4.88 gearing, a 41 tooth speedometer gear is appropriate.

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