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worth going from 3.55 to 4.10

jmsull

NAXJA Forum User
Location
Chicago
I've got a '99 4.0L, auto, 242, 3.5 lift, 32" bfg at, k&n and a borla muff. I drive city 90% of the time. I get 13 mpg avg.

I've searched, and the consensus is that 4.10 would give me better milage than the 3.55's, along with a little more power. I'm happy with the power, I just want better mpg.

My question is how much better will the mgp be?

I figure it will cost me $800 out the door to regear both ends. Thats going to take a long time to pay off with gas savings. Is it worth it?
 
Are you sure you are only getting 13mpg? Is that read from the odometer or your calculated mpg as the speedo is not accurate with the 32's... I have the same size tire as you and went to the 4:10's... It made a huge difference in power and performance, mpg didn't change much from the "calculated" to the actual.. I get 15-17 most of the time and drive pretty fast... Since I am mostly highway I didn't want to go with a lower gear than that... If it wasn't my dd I would have opted for maybe 4:56 or so,,, in the city, you could probably go lower if you wanted.... Mine was struggling on the hills at highway speeds...
 
4.56s, especially if you plan to go bigger. if you keep your foot out of the pedal you should see an improvement
 
I had 32's w/ 3.55's for a couple years. I swapped in a junkyard D30 w/4.10s and threw some used 4.10's in my D44 and I love it. It is a huge improvement over the 3.55's, but if I was going to be buying new gears and having them installed at a shop I would go lower.

Gas mileage is a solid 4 mpg better.
 
Ramsey said:
4.56s, especially if you plan to go bigger. if you keep your foot out of the pedal you should see an improvement

Like Ramsey said.....go with 4.56 gears if you are going to spend the time and $ to have your gears done. Do it once and be done with it. We all know how inchitis works and you are probably going to want to go bigger in time. Even if you do stick with your 32's the 4.56 gears will be o.k. with your AW4 and it's overdrive. I have 5.43 gears and 37's with my AW4 and I can do 80 mph and be at 3K RPMs which isn't too bad. Gear swaps are expensive and you don't want to have to do it multiple times, 4.10's are nice, but not a very big improvement over the 3.55 gears you already have. If you went from 3.07s to 4.10s with a manual you'd notice it more, but the auto has a hard enough time searching for a gear with big tires and not deep enough gearing.

Troy
 
Big Red said:
Like Ramsey said.....go with 4.56 gears if you are going to spend the time and $ to have your gears done. Do it once and be done with it. We all know how inchitis works and you are probably going to want to go bigger in time. Even if you do stick with your 32's the 4.56 gears will be o.k. with your AW4 and it's overdrive. I have 5.43 gears and 37's with my AW4 and I can do 80 mph and be at 3K RPMs which isn't too bad. Gear swaps are expensive and you don't want to have to do it multiple times, 4.10's are nice, but not a very big improvement over the 3.55 gears you already have. If you went from 3.07s to 4.10s with a manual you'd notice it more, but the auto has a hard enough time searching for a gear with big tires and not deep enough gearing.

Troy

x2
 
Big Red said:
Like Ramsey said.....go with 4.56 gears if you are going to spend the time and $ to have your gears done. Do it once and be done with it. We all know how inchitis works and you are probably going to want to go bigger in time. Even if you do stick with your 32's the 4.56 gears will be o.k. with your AW4 and it's overdrive. I have 5.43 gears and 37's with my AW4 and I can do 80 mph and be at 3K RPMs which isn't too bad. Gear swaps are expensive and you don't want to have to do it multiple times, 4.10's are nice, but not a very big improvement over the 3.55 gears you already have. If you went from 3.07s to 4.10s with a manual you'd notice it more, but the auto has a hard enough time searching for a gear with big tires and not deep enough gearing.

Troy

Listen to Troy. Even if you don't think you will go bigger than 32s 4:56 are better. I have 31s with 4:56s. Maybe a little extreme but I going to 33s so... Gears are expensive, you don't want to do them again.

Glen
 
Thanks for all the replies.

First, I have one of those Jet speedo adjusters, so the oddometer is accurate. I'm hit E before I get to 200mi.

Second, I'm well aware of inchitis, but I've had this set-up for over a year and I'm really happy with it. To go any bigger would require so many other mods and $$$$ that I'm really not interested.

With that said, would you still suggest going lower than 4.10? If I go 4.56 and never lift it any more will I regret going so low? Are there any downsides to going down to 4.56, instead of just going to 4.10?

And the big question is, what will give me the best mpg. Remeber mostly city driving 90%, some hwy 9.5%, and nowhere near enough offroad 0.5%.
 
I ran 4.56 with 32's for a long time, and am glad I didnt do 4.10. You don't realize just how much power you lost to the 32's until you gear to get it back, and I think at 4.10 you'd still be struggling on the hills and getting up to speed.
 
kleake said:
Are you sure you are only getting 13mpg?

Except for being on 31" tires, I'm getting only marginally better fuel economy (14mpg) in my 2000 Limited with pretty much the same specs on the lift and 3.55:1 gears.

Is that read from the odometer or your calculated mpg as the speedo is not accurate with the 32's...

I don't know if he did this or not, but I went to the 32-tooth speedo gear the day the tires went on. Numbers have been verified against amount of fuel bought and distance driven as well as checking the speedo against GPS. Best in regular commuting: 14.4mpg; worst, 13.6. 14mpg is the consistent average.

I have the same size tire as you and went to the 4:10's... It made a huge difference in power and performance, mpg didn't change much from the "calculated" to the actual.. I get 15-17 most of the time and drive pretty fast... Since I am mostly highway I didn't want to go with a lower gear than that... If it wasn't my dd I would have opted for maybe 4:56 or so,,, in the city, you could probably go lower if you wanted.... Mine was struggling on the hills at highway speeds...

The 4.10s likely helped a lot. I used to average 16mpg in town with the stock tires and 3.55s, and it sounds like you're back to where most of us were.

Looking at the cost of regearing for a moment, we'll stick with that $800 figure and assume that gas is at $3.30/gallon. A 2mpg improvement at 14mpg will save 40 miles of fuel per tank. At 16mpg, this is 2.5 gallons per tank saved, or $8.25. Given those numbers, you'd need to go through 97 tanks of gas to break even on the 4.10s in terms of fuel costs.

I can go through probably 70-75 tanks in a year... So while it would take time to recoup the investment, figure that in 18 months you'd finally be ahead. Of course, this is assuming you don't go to bigger tires or even deeper gears, but that's outside of the circumstances we're talking about here.
 
casm is right.... I like my 4:10's, they actually put me just a tad under stock overall gearing and RPM. I had a ring gear bolt back out and get into the teeth in my gears, so, I was replacing them anyway and could do the install myself so cost wasn't such a big deal. According to my GPS, my speedo at 70, I am actually running 66... It pretty much put me back close to what I had at stock, and for as many highway miles I drive, it's perfect. For around town, and if you don't plan to do any bigger tires, it would take a while, but in the long run it might be worth it.....
 
so you're saying 4.10 would do the job, no need to go to 4.56?
 
i think its all a matter of opinion. my being at 4.56 with 32's i thought was VERY close to stock, but im talking in performance and driveability .. i cant speak for math on paper.

I was getting about 17 mpg.. 20 on a good day.
 
My $.02




I went from 3.55's to 4.10's /w/ AW4 I only run 30's though. This happened when I swapped in the 8.8. It had 4.10's in it so I just regeared the D30. I love the 4.10's. The biggest tire I would probably ever run would be 33's and that's quite a ways down the road. Won't be ideal I know, but better than 3.07's. My average MPG is about 14-15 mostly in town driving. As far an RPM's....I don't know. I don't have a tach. It's not sceaming though.

Now, if I was in your boat jmsull, I would go with 4.56's.
 
jmsull said:
so you're saying 4.10 would do the job, no need to go to 4.56?

All depends on what your need is..... For my driving style, and being on the highway most of the time,,, 4:10's work fine,,, if you are at slower speeds or more off road, 4:56's might be better... Also, depends on future plans,, if you might go to a bigger tire,,, 4:56's would be better, but if not? Both will be an increase over what you have now... There are several charts around that give the details of RPM compared to tire size. Look at what you had at stock tire size, then compare to what you are running... Here is one...

http://www.superioraxle.com/htmlfolder/gearcalculator.html
 
its important, when looking at the rpm's you will run with those charts, that they assume you will have no overdrive.
 
so it seems that even though I'll have almost 1000 more rpms w/4.56, than I would w/3.55, I'll get better MPG.

I would imagine this is because I'll have to give it less gas to keep it at speed. In wich case go as low as possible so the engine is doing less work, and using less gas. (me thinking out loud)

Does this sound right?

So does anyone have 32" tires w/ 4.56 that does a lot of city driving, and what kind of mpg are you getting?
 
I was getting about 15 city and about 18 highway on average. I do get abnormally high mileage for a jeep of my age (1987), but the renix jeeps seem to be a bit more fuel efficiant than the newer ones, have a higher overdrive, mine only had 30,000 miles on it at the time (51k now) and im very light on the throttle.

I'm getting about 14/19 city/highway now with 4.88's and 35's.
 
ok i was in the same shoes as you a week ago when i got 4.10s for 31s on my 95 xj auto 4.0 h.o. and was getting 11 to 12 mpg i did the gears and now at 15 city and 18 highway being not so polite with the throttle so i would do 4.56 with 32s for city city driving mostly and you can do 33s next time but on the free way doing 80 i am only at 2600 and at 65 i am at 2300 so in my opinion i would stick with 4.56 and if you have a d30 in the front make sure to buy a carrier with 3.73 or higher they have a size break at 3.55 and 3.73 you need a 3.73 carrier. precision gear has them for about 65 dollars.
 
DurbElites said:
ok i was in the same shoes as you a week ago when i got 4.10s for 31s on my 95 xj auto 4.0 h.o. and was getting 11 to 12 mpg i did the gears and now at 15 city and 18 highway

This is exactly what I wanted to know, thanks
 
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