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Discussion Starter #1
Im new to jeeps I have a 88 yj 4.2 258 1 barrel 5 speed front Differential is a dana 30 3.73 and the rear Differential is dana 35 3.55 I have 33x12.5x15 tires on it now might go to 35 the fifth gear is no good on any incline I get 6.1 mpg now what gears should I put in to get better mpg and fifth gear back so I can drive down the highway
 

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You also want to make sure you have the same ratio in the front as in the rear differential.
 

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If you are going to stay with the 33" tires. Run a 4.10 ratio. If you are going to 35" tires, then 4.56.

If you want fuel mileage above 10-14, go back to a stock tire. Or try running it without mashing on the gas pedal. The 4.2L is a low end motor. Meaning that the RPMs should never exceed 2500. Anything above that and you are just wasting gas.

I run 4.56 gears with 35" tires. 87 YJ 4.2L, 5spd. and the best mileage I have gotten since I put a fresh motor in it was in the 12MPG range. MPG just isn't there with a 4.2L and large tires. Isnt going to happen.

And like Klug said, the ratio in the front and the rear have to match if you want to use 4wd. If you do have a 3.73 in the front and a 3.55 in the rear, then you will have a broken t-case after engaging 4wd.
 

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Discussion Starter #5
will the 4.56 be good with the 33" tires 35" tires are a little ways off I would be happy with 12 mpg that's like double what I get now 6.1 just blows lol I don't want to regear now and then again later will never put stock tires on it again and cant help it I have a heavy foot
 

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I put 4.10's in my 91 YJ with the 6 cylinder engine. Didnt notice a difference until I dropped in 4.56!
 
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