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MeSoStupid
Dodge Dakota
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4/27/2003
05:35:51
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 Subject: Someone explain gear ratio to me please.
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I may ove ot drive, but i don't know $hit when it comes to auto mechanics. What does it mean to have 3.55 gears, or 3.9...what ever there is. What do the different rations do for the truck?
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Evil Doer
Dodge Dakota
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4/27/2003
07:25:51
| RE: Someone explain gear ratio to me please.
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Ok, here goes.
Gear ratio refers to the gears in the rear axle. A 3.55:1 ratio means that for every 3.55 revolutions of the driveshaft, there will be 1 revolution of the tire.
A lower ratio (3.92, 4.11, etc) will give you better acceleration off the line but your engine will rev higher at highway speed.
A higher ratio (3.55, 3.23, etc) will not pull as hard from a dead stop but will allow you to achieve higher top speed and a more reasonable engine speed on the highway.
Generally, a 3.55 ratio is considered a good compromise for all around driving conditions for a typical 2WD truck. A 4x4 would prolly want 3.92's or lower because of the taller tires.
If you drive mostly on the street & highway but occasionally want to drag race your truck, try running smaller tires in back at the track. It's the same as changing to a lower gear without tearing the rearend apart.
Hope this answers your questions.
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Mike Carver
Dodge Dakota
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4/28/2003
07:44:42
| RE: Someone explain gear ratio to me please.
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Ok, that was a great explanation, thanks. Here's my problem: I just went up a tire size after a body lift, to 33s on my 4x4 Dakota, 3.9l engine. This weekend was the first time I had it out on the highway, and I was having a lot of difficulty getting up to higher speeds, and my odometer, and speedometer were off. How do I correct this?
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Evil Doer
Dodge Dakota
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4/28/2003
21:59:30
| RE: Someone explain gear ratio to me please.
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By going to the taller tires you raised the gear ratio (numerically) and now aren't able to accelerate as fast, and the V-6 is struggling to turn the taller tires with the higher ratio. The only way to correct that is by changing the gears to a lower ratio.
As for the odometer and speedo, if you change to a lower ratio that brings it back near the factory ratio, taking into account the taller tires, it will correct the problem. If you're truck has the 3.55 gears, changing to 3.92's should get it back close to like it was before.
Only other way is to have the PCM recalibrated with the new specs.
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Michael Carve
Dodge Dakota
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4/29/2003
10:55:20
| RE: Someone explain gear ratio to me please.
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Thanks. That helps. My dealer told me that the PCM can't be reprogrammed, that it was not an option on my truck. I guess I'll have to consider the lower gears. Thanks.
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Evil Doer
Dodge Dakota
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4/29/2003
16:48:07
| RE: Someone explain gear ratio to me please.
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No prob. Found this old post, maybe it will help some.
A simple equation for finding out tire changes have on gearing: (new tire/old tire x rear gear= the result the new tires have on what the gearing feels like). To find the gear ratio required to fix the speedometer reading(old tires/new tires x stock rears= ideal ratio).
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Evil Doer
Dodge Dakota
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4/29/2003
16:54:17
| RE: Someone explain gear ratio to me please.
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I think the formulas are backwards above.
Assuming stock tire size was 31" and factory ratio was 3.55, then your truck 'thinks' it has a 3.33 gear.
A 3.77 would get it back in spec, but it doesn't exist, so 3.92' are gonna be your best bet I think.
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