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-   -   Gear caluclation? How to keep ratio's same as factory with big tires? (http://www.wranglerforum.com/f210/gear-caluclation-how-to-keep-ratios-same-as-factory-with-big-tires-16769.html)

branty 03-18-2008 03:17 PM

Gear caluclation? How to keep ratio's same as factory with big tires?
 
I know this subject has been worn out time after time but I have to ask anyway. Is there a way to caluclate exactly what gear ratio I would need for different size tires in order to be close to factory conditions. I have seen a few charts but I did not know how to read them. I have read many peoples opions on what gears would be best for certain size tires based on their engine and transmission. However, I would like to know how to calculate what gears I need to buy if I stay with my current tire size or if I decided to step up a size.

To be a little more specific, I posted a few days ago and everyone agreed that I could fit 35's on my Jeep. I want to go this route but know I will have strengthen the axles with a kit and change gears. I am thinking about the super 35 kit and don't know what gears to go with. I am not trying to have 3 crawl gears and 2 overdrive gears, I would like to have 5th gear back on the highway and be able to crawl?

Forgot to add: 4.0 and 5 speed manual, currently 33's with 3.07

Thanks for the input.

compshooter 03-18-2008 03:37 PM

http://www.4lo.com/calc/geartable.htm

All you want to know.

Jerry Bransford 03-18-2008 04:13 PM

One thing to keep in mind, which is a very important thing. Gear charts that show RPMs only tell you part of the story and if you use nothing but a chart, you'll be disappointed with the outcome. You actually need to regear to obtain slightly higher RPMs at the same speeds with bigger tires than you had previously with smaller tires.

This is because the bigger tire is harder to turn for these reasons... it weighs more, it has increased rolling resistance, the larger diameter has a longer radius that gives it greater leverage against the axleshaft trying to turn it, and that a lifted Jeep with big tires sits higher up in the wind stream so it has to fight harder to push itself down the highway at 65-70 mph.

So don't be afraid to go with a ratio that gives slightly higher RPMs than you had before, you'll need them. No one ever complained they regeared too low but plenty, me included after regearing to 4.10 for 33" tires, have complained they didn't gear low enough. That's why I went to 4.88 for 35" tires, I was sick of being undergeared. :)

bpowa 03-18-2008 05:03 PM

Quote:

Originally Posted by compshooter (Post 208342)

i think that chart is wrong.. I have 3.73 gear with 32' and I am cruising at 1900-2000 rpm at 65mph.

Jerry Bransford 03-18-2008 05:17 PM

Quote:

Originally Posted by bpowa (Post 208384)
i think that chart is wrong.. I have 3.73 gear with 32' and I am cruising at 1900-2000 rpm at 65mph.

Read it a little more carefully, RPMs are given for a final drive ratio of 1.0:1, not what a transmission that has an overdrive final drive ratio would give.

That chart's RPMs would match my new 3-speed automatic's RPMs since it does not have overdrive, but it would not match my previous 5-speed transmission if I was in 5th gear... but it would if I downshifted to 4th gear which is 1.0:1. :)

bpowa 03-18-2008 05:23 PM

I have a 4spd, so for optimal mpg on the freeway I should not use overdrive?

Jerry Bransford 03-18-2008 06:16 PM

Quote:

Originally Posted by bpowa (Post 208410)
I have a 4spd, so for optimal mpg on the freeway I should not use overdrive?

No, you should use overdrive for best fuel economy. Where did you get that you should not? Re-read what I said, I only said that your RPMs are while you're in overdrive, that chart is showing RPMs without the use of overdrive so they will be higher than your RPMs. :)

JCS05Rubi 03-18-2008 10:03 PM

that chart is showing the gear ratios for direct drive, not over drive. If you have a 3spd auto, the chart is correct for 3rd gear.

4spd auto, the chart will show your rpms in 3rd gear, your 4th gear rpms will be lower based on your overdrive ratio in the trans.

5spd manual the chart will show your 4th gear rpms, 5th will be dependant on ratio

6spd manual the chart will show your 5th gear rpms, 6th will be dependandt on ratio..

etc. etc. :D


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