MPG Question

Fulltimer · **Posted:** Tue Jun 17, 2008 3:49 pm

You can see in my sig. line the tires I'm running. They are 31.7" high. When I put them on I lost MPG, stands to reason. When I put 4.10's in it I will go back close to the original MPG. Am I right that with the 4.10's in place and I go back to stock sized tires I will again loose gas mileage?

Terry

tjkj2002 · **Posted:** Tue Jun 17, 2008 3:54 pm

Quote:

Am I right that with the 4.10's in place and I go back to stock sized tires I will again loose gas mileage?

Yes and no.

You will lose mpg's on the highway but might actually gain mpg's in the city due to it being easier to get the KJ moving with 4.10's then 3.73's,but the higher rpm's at highway sppeds will decrease the mpg's by alot.

I'm in the same situation as you with my SFA swap,4.88 gears is what I really need to be back in the stock range(with 35" tires) but I'm going to 5.13's for better city mpg's(which I drive in town more than highway),I'll just have to slow down some on the highway.

Fulltimer · **Posted:** Tue Jun 17, 2008 7:27 pm

That is what I was thinking. I'll be able to take off faster, not that I would mind you, but for interstate travel it will suck. The speedo will be okay again right?

Terry

Jeepjeepster · **Posted:** Wed Jun 18, 2008 11:50 am

With a tire that much bigger over stock, how many more rpms would be be turning at hw speeds? I wouldnt think it would be that much.

tommudd · **Posted:** Wed Jun 18, 2008 12:20 pm

If you stay with your present tires at 31.7s tall and go to 4.10s you will be real close to what it was stock with a 29 inch tire and 3.73s, but yes if you go back to the 29 inch tire and 4.10s you will be running much higher RPM out on the open road would be close to 400/450 or so RPMs more just calculating in my feeble mind :wink:

Fulltimer · **Posted:** Wed Jun 18, 2008 1:36 pm

Decisions...decisions! A guy down here is checking on prices for 4.10's for me. He thinks $230 to $250 for the front with no shipping charge. Not a big surprise that he has NEVER done a KJ, and building differentials is his job! I have a quote from that new company on here for front/back and both install kits and it's close to 1K with shipping. When my wife hears that I'll have room for a second floor on the house....she will really raise the roof! Or I may be looking for a home for me and "Trail Buster".

Terry

tommudd · **Posted:** Wed Jun 18, 2008 1:42 pm

I know what you mean about "raising the roof" :lol:

(some)women just don't understand that we men "need" certain things in life to keep us happy :wink:

I am looking as well for 4.10s and installation for my rear diff, have a whole new front diff with 4.10 gears already installed laying in the garage just waiting....... :roll:

Jeep Dawg · **Posted:** Wed Jun 18, 2008 3:34 pm

where are you guys getting the 4.10's?
doing both front and backs?

Inc · **Posted:** Wed Jun 18, 2008 3:42 pm

Question? Why do the front at all, since the kj is a rear wheel drive? I have no idea what I am talking about (as if you guys haven't noticed) but it seems to me that since the kj is rear wheel drive, you would only need rear, or would that really screw things up? Help out a clueless kjer. Thanks

Fulltimer · **Posted:** Wed Jun 18, 2008 3:49 pm

Yep, front and back. I have a locale shop that came up with the $230 to $250 for the front. If you look in the vendor section you will see a thread for a new (to our forum) vendor that carries, or can get, 4.10's.

Tom: why is it that other manufacturers don't make 4.10's that we can use? Is it a lack of market volume? And...why don't they make more selections for the KJ?

Terry

tjkj2002 · **Posted:** Wed Jun 18, 2008 3:51 pm

incubusmike wrote:

Question? Why do the front at all, since the kj is a rear wheel drive? I have no idea what I am talking about (as if you guys haven't noticed) but it seems to me that since the kj is rear wheel drive, you would only need rear, or would that really screw things up? Help out a clueless kjer. Thanks

If you do not change both front a rear gear ratio's you can kiss your t-case,diffs,and driveshafts goodbye from the massive binding that will take place from the front and rear diffs spinning at severely different speeds.Unless you run 32" tires up front and 28"-29" tires in the rear you will do major damage.

With that being said the front gears are not a true 4.10,they actually are 4.09's with the rear being a true 4.10 ratio.A slight,and I mean slight difference is needed to keep you going straight in the slick stuff while in 4wd.

Quote:

have a quote from that new company on here for front/back and both install kits and it's close to 1K with shipping

That's a major rip off,the rear gears and install kit will run about $280-$300,the front gears and install kit should only run about $300-$350.He charging $400 for shipping?

tjkj2002 · **Posted:** Wed Jun 18, 2008 3:55 pm

Quote:

why is it that other manufacturers don't make 4.10's that we can use? Is it a lack of market volume? And...why don't they make more selections for the KJ?

Precision gears just started making D30A gear sets,only 4.10's though,and of course the Dana/Spicer gear set's can be had from Rieder Racing.There is some cheap china made D30A gear sets out there,watch out for those,they are made of real weak steel and are no good,get a good gear set from Precision or Dana/Spicer.

Fulltimer · **Posted:** Wed Jun 18, 2008 4:03 pm

I was quoted $975.00 with shipping. I can do better around here. But, what is the reason they don't make lower gears than the 4.10's. Something in the upper 4's or even 5's?

Terry

Cobalt-60 · **Posted:** Wed Jun 18, 2008 4:52 pm

tjkj2002 wrote:

Quote:

why is it that other manufacturers don't make 4.10's that we can use? Is it a lack of market volume? And...why don't they make more selections for the KJ?

Precision gears just started making D30A gear sets,only 4.10's though,and of course the Dana/Spicer gear set's can be had from Rieder Racing.There is some cheap china made D30A gear sets out there,watch out for those,they are made of real weak steel and are no good,get a good gear set from Precision or Dana/Spicer.

Who makes the ones that Crown Automotive sells? Trailquest has the Crown 4.10 Dana 30a Ring & pinion (#5066085AA) for $216.95 plus shipping:

http://www.trailquest.com/axles/KJ_Model30.shtml

Troy, are these some of the ones that you are warning about?

tjkj2002 · **Posted:** Wed Jun 18, 2008 5:00 pm

Cobolt-60 wrote:

tjkj2002 wrote:

Quote:

why is it that other manufacturers don't make 4.10's that we can use? Is it a lack of market volume? And...why don't they make more selections for the KJ?

Precision gears just started making D30A gear sets,only 4.10's though,and of course the Dana/Spicer gear set's can be had from Rieder Racing.There is some cheap china made D30A gear sets out there,watch out for those,they are made of real weak steel and are no good,get a good gear set from Precision or Dana/Spicer.

Who makes the ones that Crown Automotive sells? Trailquest has the Crown 4.10 Dana 30a Ring & pinion (#5066085AA) for $216.95 plus shipping:

http://www.trailquest.com/axles/KJ_Model30.shtml

Troy, are these some of the ones that you are warning about?

You makes them,What brand? If it's not Dana/Spicer or Precision gears I would run far away from them,the other good gear makers do not make D30A gears.

Cobalt-60 · **Posted:** Wed Jun 18, 2008 5:05 pm

tjkj2002 wrote:

Cobolt-60 wrote:

tjkj2002 wrote:

Precision gears just started making D30A gear sets,only 4.10's though,and of course the Dana/Spicer gear set's can be had from Rieder Racing.There is some cheap china made D30A gear sets out there,watch out for those,they are made of real weak steel and are no good,get a good gear set from Precision or Dana/Spicer.

Who makes the ones that Crown Automotive sells? Trailquest has the Crown 4.10 Dana 30a Ring & pinion (#5066085AA) for $216.95 plus shipping:

http://www.trailquest.com/axles/KJ_Model30.shtml

Troy, are these some of the ones that you are warning about?

You makes them,What brand? If it's not Dana/Spicer or Precision gears I would run far away from them,the other good gear makers do not make D30A gears.

I don't know. They are distributed by Crown Automotive (http://www.crownautomotive.net/catalog/ ... l30FKJ.htm) but I was asking if you had any idea who made them? :wink:

tommudd · **Posted:** Wed Jun 18, 2008 5:06 pm

my front diff thats laying in the garage has the Pricision gears in it, just trying to find the best price for the rear now.

Terry, your question is lack of sales I would say. I think we "may see" an increase in available gear ratios etc as these things get cheaper to buy and wheel but then maybe not
:roll:

who knows

But with 4.10s and a 32 inch tire it should be pretty good ...I hope! At least thats where I am heading for a year or two then ..... :twisted:

bigger no matter what the price of gas is !

Fulltimer · **Posted:** Thu Jun 19, 2008 12:56 pm

Well, I just heard back from my locale guy: $1,150.00 for front and back!

Terry

tommudd · **Posted:** Thu Jun 19, 2008 1:51 pm

Fulltimer wrote:

Well, I just heard back from my locale guy: $1,150.00 for front and back!

Terry

is that parts / labor and all?
If you could remove your front right there they are hitting you for at least 6 or 8 hours of labor. Its not hard to do (to remove the front diff)

tjkj2002 · **Posted:** Thu Jun 19, 2008 1:51 pm

Fulltimer wrote:

Well, I just heard back from my locale guy: $1,150.00 for front and back!

Terry

Just gears and install kits? Or is that including install labor also?

If that includes install labor that's not bad at all since you got to remove the front diff to swap the gears.Call Reider Racing at 1-800-522-2707 to get prices for the gears---http://reiderracing.com/

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