changing my gear ratio

mercab18 · #1 (**permalink**) 05-28-2008, 09:42 PM

since i've gotten my jeep, i've put in a 2.5" lift with 33" tires. i was hoping someone could give me some info about gear ratios.

-how are they installed? can i do it myself? how much should it cost?
-will it make my gas mileage better?
-will anything else have to be replaced?

any information is greatly appreciated, thanks everyone

currupt4130 · #2 (**permalink**) 05-28-2008, 09:44 PM

Someone else please field this. I'm too tired tonight.

The short and dirty.

Yes you can, but good luck unless you're very good with mechanical stuff. And I mean very good.

Yes.

Sort of.

jps4jeep · #3 (**permalink**) 05-29-2008, 08:01 AM

I got yer back..

Unless you are planning on installing a number of different gear sets, the cost of the "proper" tool will be about what it cost in labor to have someone else do it, if not more. Yes you can install gears with less than "proper tools" but makes life hell.

Depending on how well your initial set up is, you might need to press on and off the pinion bearing a number of times, and with out proper collars and press this can be a PITA, sure you could do the over/freezer method and bang the bearing on with a hammer and tube doly, but getting it off if you need to re-shim is tough, and could get expensive if you need to make 4-5 trips to the machine shop to do it.

here is a good web site to look over, they really do a good job of explaniing the tools and process

Differential Repair & Installation Instructions

Lastly, the best thing of having a shop do the install (which I learned the hard way the first time I swapped gears) they warranty work should something happen.

To answer the other questions quickly, your milage may actually suffer depending on which gears you install, the Idea is to find the optiomal rpm's at the optimal speed to optimal efficiency, and there will be nothing else you need to replace out side the diff.

jfrabat · #4 (**permalink**) 05-29-2008, 10:22 AM

Quote:

Originally Posted by jps4jeep

I got yer back..
To answer the other questions quickly, your milage may actually suffer depending on which gears you install, the Idea is to find the optiomal rpm's at the optimal speed to optimal efficiency, and there will be nothing else you need to replace out side the diff.

I always thought that if you put larger tires, mpg went down because you were putting more load on the engine. If you bring your relation back to stock, mog should go up, not down. If you go over the stock relation (more torque) then mpg should go back down, but it depends on the relation...

In my case, with the 4 popper, 33" tires and 4.88 gears should "theoretically" give me better gas mileage than 33" tires and 4.10 gears... That's my understanding, anyway!

Felipe

jps4jeep · #5 (**permalink**) 05-29-2008, 10:56 AM

Quote:

Originally Posted by jfrabat

I always thought that if you put larger tires, mpg went down because you were putting more load on the engine. If you bring your relation back to stock, mog should go up, not down. If you go over the stock relation (more torque) then mpg should go back down, but it depends on the relation...

In my case, with the 4 popper, 33" tires and 4.88 gears should "theoretically" give me better gas mileage than 33" tires and 4.10 gears... That's my understanding, anyway!

Felipe

There are a number of variable to consider, vehicle, driving style, location, terrain. I live in the North east, it is pretty hilly and altittude changes from sea level to thousands of feet above in matter of a couple miiles, with 33's and 3.07's, milage was less than factory but.. If I lived in say the mid-west where it is pretty flat, pretty much the same altittude, if I could keep the jeep in 5th gear for highway driving at a sustained 2100 RPM's, I would actually get better than factory milage. ya there is more roatating mass, but once you maintain speed, that is not really much of a factory.

Same vehicle I put in 33's and 4.10's from a 4 banger and I got near factory milage.

Now I have 36's and 4.88's and from what little highway driving I have done with it, it sucks.

jfrabat · #6 (**permalink**) 05-29-2008, 11:05 AM

Well, I currently live in Miami (does not get much flatter than this!) and I got 31's on 4.10's, and if it is windy outside, I get better mileage in 4th than I do in 5th... About 0.5 mpg of difference. If it is calm, then I get the better mileage in 5th (so I ussually drive in 5th with the wind, and on 4th against it).

Bounty__Hunter · #7 (**permalink**) 05-29-2008, 08:56 PM

The reason the proper gearing gets you better gas mileage, and why you get better mileage in 4th than in 5th, is because the engine is operating at it's optimum power band.

jfrabat · #8 (**permalink**) 05-30-2008, 01:52 PM

Quote:

Originally Posted by Bounty__Hunter

The reason the proper gearing gets you better gas mileage, and why you get better mileage in 4th than in 5th, is because the engine is operating at it's optimum power band.

That's what I thought... So, putting the right gear should give you better mpg (as long as you dont go lower to get more torque for the trails), right?

Bounty__Hunter · #9 (**permalink**) 05-30-2008, 03:01 PM

Correct, too low will hurt mileage as well. My 2.5L is pretty happy on the highway with 35" and 36" tires and 5.38 gears.

llovet06 · #10 (**permalink**) 10-07-2009, 12:48 AM

What kind of highway speeds can you actually hit with those 5.38 gears? What would happen if you ran that with 33"? How would the mpg be?

jfrabat · #11 (**permalink**) 10-07-2009, 09:30 AM

I would run 5.13 with 33's (I currently run 4.88 and 33's, and I was happier with 4.88 and 31's as far as gearing, which is why I recommend 5.13 with 33's). Of course, for me, 5.13 would mean 2 new axles, so I'm sticking to 4.88 for now...

camsyj95 · #12 (**permalink**) 10-08-2009, 06:58 AM

I also run 4.10s on 33's but the biggest thing I lost was speed! I got more acceleration with a lower top speed which is fine with me because I rarely hit the interstates or highways with higher speed limits. My Jeep now tops out at 55, maybe 60 going downhill with the wind pushing! I get about 14 mpg where I was getting about 16 before i got the bigger tires and gears but I wouldn't go back now that's for sure!

JohnnyO · #13 (**permalink**) 10-08-2009, 08:41 AM

Quote:

Originally Posted by mercab18

-how are they installed? can i do it myself? how much should it cost?

No offense, but if you have to ask then you probably shouldn't try it yourself. On other boards I've seen prices vary around the country, but on average it's about $300 to $400 per axle.

Quote:

-will it make my gas mileage better?

Possibly. If you get the rpm vs. road speed back to what it was stock, then you will usually get better mileage. In my experience once you go over 33" tires, then the rotating mass is much larger and mpg goes down no matter what else you do.

Quote:

-will anything else have to be replaced?

Maybe some small parts but typically not much as long as everything else is in good shape.

Ours is a 4-banger with a 3-speed automatic, I think 3.73 gears (we just got it), and 225/75-15 tires, and the engine is screaming on the highway. I plan on getting 31's and not changing the gears.

jfrabat · #14 (**permalink**) 10-08-2009, 11:43 AM

Quote:

Originally Posted by camsyj95

I also run 4.10s on 33's but the biggest thing I lost was speed! I got more acceleration with a lower top speed which is fine with me because I rarely hit the interstates or highways with higher speed limits. My Jeep now tops out at 55, maybe 60 going downhill with the wind pushing! I get about 14 mpg where I was getting about 16 before i got the bigger tires and gears but I wouldn't go back now that's for sure!

You have MORE accellaration with the same ratio and bigger tires? It should work out the other way around...

Bounty__Hunter · #15 (**permalink**) 10-08-2009, 01:16 PM

Quote:

Originally Posted by llovet06

What kind of highway speeds can you actually hit with those 5.38 gears? What would happen if you ran that with 33"? How would the mpg be?

I cruise easily down the interstate at 70-75, the 4banger loves to scream along at 3grand all day long. It would be a little too low for 33" tires, limiting you to around 60-65.

As for increasing acceleration without regearing, not gonna happen.

Collin the Hen · #16 (**permalink**) 10-08-2009, 03:25 PM

Quote:

Originally Posted by JohnnyO

On other boards I've seen prices vary around the country, but on average it's about $300 to $400 per axle.

Mine cost $200, but I took the axle to the shop with everything ready to go. The front will follow soon...

jfrabat · #17 (**permalink**) 10-08-2009, 03:32 PM

I spent a lot more on mine, but it included new axles (alloy), gears, lockers, and everything needed to put that in along with labor costs.

Collin the Hen · #18 (**permalink**) 10-09-2009, 04:42 PM

Quote:

Originally Posted by jfrabat

I spent a lot more on mine, but it included new axles (alloy), gears, lockers, and everything needed to put that in along with labor costs.

Yeah, $200 was just labor. I don't want to calculate the total cost for that rear axle... ignorance is bliss in this case.

HiHood · #19 (**permalink**) 10-09-2009, 05:04 PM

[QUOTE=mercab18;185631]

-how are they installed? can i do it myself? how much should it cost?

As a previous guy mentioned, unless you plan on doing a bunch it probably ain't worth doing it yourself. Your need Persian blue, a dial indicator w/mag base to check gear ride patterns and backlash. It takes much time with persistence and patience because you have to put everything together, check backlash, gear patterns, take it back apart, add or remove shims. That means pressing on/off the bearing. If it's your first time, it will take even LONGER and be a pain in the (censored) for certain. And after all that, it still may be done wrong.