What will new gears do exactly?

[gwayms]

I just installed a 4.5" lift with 32's. I keep reading about changing gears to 4:10 or 4:56. I know the basics (i think). But what exactly will I gain from changing them. Better gas mileage, more power. fix Speedometer, better on highway. It seems like it runs pretty good now, a little less HP but just figured that was the tires. And how much will this cost, approx.

[Mudderoy]

Your horse power hasn't changed. What has changed is the effort it takes the engine to get the mass of your vehicle up to a certain speed. Changing the differential gears compensates for the change in the diameter size of your tires.

Putting 4.10 gears in the diffs get you closer to the factory set up, when you figure in the new tire size. 4.56 goes past the factory equivalent. 4.10 is the closest (as I recall) to the factory with 32" tires.

It may get your speedo back close, but I would just go get the proper speedo gear and replace it, it's a 10 minute job, if that.

The 4.10/4.56 gears will let you accelerate easier, and faster, more like it did when you had stock tires on it. City gas mileage CAN improve since it does not require as much effort from the engine to get the vehicle up to speed. This can hurt your gas mileage on the highway because the engine turns higher RPMs at the higher speeds.

I personally didn't like my 32" tires with 3.55 gears on the highway. I would have to kick down to 4th gear (from over drive) to make it over an overpass. The 4.56 gears I have made a huge difference in how the XJ feels to drive.

4.10 was what I was originally going to go with, but I was told by several people to go one step lower (or higher depending on how you look at it) so I went with 4.56 gears. I knew I was going to go with larger tires in the future.

You won't have more power, but it will feel like it, because it will accelerate faster than it does now.

I did my diff. gears a couple of years ago. I spend $510 on parts and each axle is about $300 to $400 to have the gears set up at a shop. So I would estimate about $1500. It is one of the best modifications I have made to my Jeep.

Before you go spending that kind of money on something make sure you have the right axles. My Dana 30 front and Chrysler 8.25 rear were "marginal" for the size tires I want to go to, 35". But throwing more money at the issue should make those factory axles strong enough for the 35" tires.

[gwayms]

ok kinda what i thought. I re-read my post. Didnt mean less HP just feels a little sluggish cause of the bigger tires. So I guess now I need to deciede if I want/can spend the money on it. Did changing the gears effect any offroad performance. Mudding etc

[Mudderoy]

ok kinda what i thought. I re-read my post. Didnt mean less HP just feels a little sluggish cause of the bigger tires. So I guess now I need to deciede if I want/can spend the money on it. Did changing the gears effect any offroad performance. Mudding etc

No problem I'm a technical sort of guy so I take things at face value and try to answer the questions directly. If I start guessing what someone means, I may answer incorrectly and I do enough of that already.

Yes it will effect off road performance. Same issue as driving on the streets. With the larger diameter tires and stock gears it is harder to get the Jeep moving, or up and over rocks, or through the mud. Keep in mind you can deal with a good portion of that by dropping it into 4x4 low, but that LOW is even LOWER if you have the higher numerical, lower ratio differential gears.

The lower the gears the more "power" you get from the engine to the tires, but you give up your top end speed, and gas mileage at highway speeds.

[default83]

great explantion!

but one thing to add, the difference between 4.11 & 4.56 is not much at all, if you are planning for bigger tires in the future, id go 4.88, i have 4.11 in mine with 33's it felt like a scooter.


if you are going to run 35's in the future go 4.88. if you sticking with 33's and under 4.56 will probaly be good enough, however there is no sense in having to regear it twice if you decide to go bigger later.

[gwayms]

I dont plan on going any bigger. Im pretty happy with the lift and tires now. 4:56 is what everyone seems to say is right for me.

[Mudderoy]

great explantion!

but one thing to add, the difference between 4.11 & 4.56 is not much at all, if you are planning for bigger tires in the future, id go 4.88, i have 4.11 in mine with 33's it felt like a scooter.


if you are going to run 35's in the future go 4.88. if you sticking with 33's and under 4.56 will probaly be good enough, however there is no sense in having to regear it twice if you decide to go bigger later.

I think there is only one company that makes 4.88 for C8.25, but they do have them now. Also going with 4.88 might be good for off road, but I'm having a running hot problem with my 32's and 4.56 when I get on the highway. With all that I have been through trying to resolve this issue I suspect it is due to the heat generated from then engine running at 2800 rpm driving down the road at 70 mph. Even with 35" tires and 4.56 gears I will won't be back to stock gearing ratios.

So I agree with what you are saying, but you have to take into account whether the rig is mainly off road, street, or both.

[4.3LXJ]

I think there is only one company that makes 4.88 for C8.25, but they do have them now. Also going with 4.88 might be good for off road, but I'm having a running hot problem with my 32's and 4.56 when I get on the highway. With all that I have been through trying to resolve this issue I suspect it is due to the heat generated from then engine running at 2800 rpm driving down the road at 70 mph. Even with 35" tires and 4.56 gears I will won't be back to stock gearing ratios.

So I agree with what you are saying, but you have to take into account whether the rig is mainly off road, street, or both.

If you go with 4.10 gears, the front will be cheap. Just get a cheap D30 from a 4 cyl XJ and swap the whole axle. Then all you have to do is change the rear. If it is mainly highway driving you do, then I suggest the 4.10s. If you are thinking trail monster then go with 4.56 or 4.88

[gwayms]

I mainly do highway driving. Every once in a while offroad, but its my daily driver.

[4.3LXJ]

I would go with the 4.10s then if it was mine. 4.27 would be better but are not available for the XJ axles.

[troys96xj]

It will return lost power and your mpg.

[JRW7072009]

I agree I would do 4.10s unless you want to go with bigger tires then do 4.56s

I got a nice power gain from doing my gears I went from 3.55s to 4.88s with 35" tires.

It used to struggle up long hills and when I went wheelin and now it has plenty of power up hills and I dont use 4 low nearly as much when wheelin.

I also went from 9mpg to 16mpg with doing my gears.

So IMO gears are a very good thing to do.