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Hello,

I need help from those who know more than me. I'm swapping in a Ford 8.8 and all the literature suggests a 4:10 gear ratio. Those are harder to find I'm finding out. I have easy access to a 3:73. I've been told that I will lose some power with that setup. I'm running 3:55 stock with 31's right now. I know that I will have to do an install on the front end before I can engage 4wd so I will probably wait until I swap in the common D44. I'm building my Jeep slowly so it's more of a daily driver than a serious trail vehicle right now although I'm working on it. I plan on running 33s or 35s some day so... what is the optimal gear ration for a trail/street vehicle, what exactly does "loss of power" mean in terms of rpm to mph, etc. Should I go with the easier to find 3:73, 3:55 to match my current stock setup, or hold out for the 4:10. Time is a bit of an issue, I'm tired of being without my Jeep - I need to finish this project.

Thanks for your input in advance.

-JL

91 YJ
4.0 liter
31 inch tires with plan to grow at next buy
Slip yoke eliminator installed (hence the need for major rear end work)
 

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What transmission do you have? It's not clear from you profile, although I would guess it's a manual shift four-speed.

If you go to 3.73 axels and 33's you'll have about the same gearing as now. If you put on 35's you'll find it a little harder to get rolling from a stop, especially starting out uphill. For street use it would be marginal. Off road you would probably find the gearing to be a handicap.
 
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Rule of thumb (not suree who Thumb was, but he had a lot of rules)

32-33" tires = 4.10 gears
35" tires = 4.56 gears

This assumes stock transmission

If you are running an automatic, then you can get by with a slightly lower ratio....

You have a tough decision....if you know your going to go to larger tires, then what you do depends on how long it will be.....gears are one of those things that you can't exactly sell off to someone else later......
 

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The transmission is very important. If it is has over drive but a high first then deeper gears are often better. If it is a 4 speed if no OD but a deep first gear then shallower axle gears can be good as they can give the same results.

This is the logic or lack there of that I am using in my current buildup.

Stock:
Tires: 205/75r15 which are about 27" tall.
Transmission: AX15 which has a .79 OD and a 3.83 first.
Axles: 3.08 gears.
Transfer case: NP231 2.72 reduction.

Built up:
Tires: 35x12.5r15 which have an operating height of 34".
Transmission: NP435 which has a no OD and a 6.68 first.
Axles: (3.54) gears.
Transfer case: Dana 300 2.62 reduction.
Crawl Ratio: 3.54*6.68*2.62=62

So I first looked at the stock setup.
Stock:
High Gear reduction including OD: 3.08*.79=2.43
Crawl Ratio: 3.83*2.72*3.08=32

Now we need to do something to include the effect of tire height. The taller the tires the more crawl ratio you need. So lets divide High by tire size and then multiple by the new tire size to remove the effects of changing tires.

So New:
High Gear reduction: 2.43/27*34= 3.06

This means that to have the same engine rpms in high gear I would need an high gear of 3.06. If I had OD I would divide 3.06 by the OD to get my axle gears.

I chose to go with 3.54's for several reasons. First I will have more air resistance with the lift and tires and will need more power at highway speeds. Second one problem with the NP435 is it has a much bigger gap between 4th and 3rd then the AX15 has between 5th and 4th. So on the hills that I have been down shifting on with the AX15 I have a slightly lower gear which allows me to make it up the hill. With the NP435 I will have to down shift much more when going to 3rd, so I have tryed to make that less needed by having a little deeper gear in 4th.

I when doing both a tire and transmission swap I would recommend making a spreadsheet with old and new configurations and play with different options. Look not only at low and high gears but also at shift points.

If you PM me your email I will send you the spreadsheet I have setup.
 

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If you plan on doing serious off roading I would go with 35" tires and 4.56 gears. If you are wanting a street machine go with the 3.73 gears and the 33" tires. With 4.10 gears and 33" tires you have a well rounded on --- off-road jeep.
 

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Well, the pinion gets smaller diameter but the ring gear gets thicker. /ubbthreads/images/graemlins/tongue.gif
 
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