I have searched and found all kinds of threads concerning new gears but none that gave me the info I was looking for, I hope you all can help.

I am looking to regear, I have a 6 speed, 3.21's and have 33" tires, I figuire to go back to stock I need 4.10"s. The question would be if I went even lower would it make gas mileage worse or better. The extra power would be nice but at what expense would it be?

 

http://www.superioraxle.com/feature.html
Gear Calculater from superior gear and axle

 

That's a tough one to call. Since the 3.21s require a lot more power to overcome inertia with the larger tires, that's using more gas than when it was stock. Once it gets to highway speed, it's using less because the larger tires raise the effective gear ratio, and slow the engine down. Again though, acceleration for passing and hill climbing probably negates a lot of possibly all of any savings gained by the larger tires. Did you buy the Jeep for fuel economy? The 4.10s will pick it up significantly, but what affect they'll have on mileage is anybody's guess since there are so many other factors involved. I would never have bought a Jeep with 3.21s in the first place. But that's me.

 

Being new to the Jeep world I did not know any better than to get the 3.21's......But I am wiser and poorer now. As far as buying it for fuel economy...no I did not, infact it does better than the Titan I traded in for it. I do however drive about 75-100 miles a day and I want enough power to use 6th again and to be able to pass on the highway. I know the 4.10's will get me back to where I was at stock, I am thinking the extra power of 4.56's would be nice, but if it will kill my MPG than I don't want to do it.

 

if you're thinking of upgrading, might as well go with 4.56. I'm thinking of doing this, but since i already have 4.10 from factory, it's not worth it.

But 4.56 will be a great upgrade for 3.21 if you're only planning on running 33". If you're thinking of going to 35" in the future, i would say wait until then. Then, you can get 4.88

 

For what it's worth, I went from 34's w/ 3.21's and a 2" lift to 37's w/ 4.88's and a 4" lift and only lost 1 mpg. Lower gears will help, but if you go too low, it can and will hurt fuel economy.

I personally don't think it's worth the expense if you don't plan on going any bigger, but that's just me. If you do, skip the 4.10's and go w/ the 4.56's unless you think 35 or bigger tirers are in your future.

 

it seems to me if you decide on 410 id buy the rubi axles rather than regear

 

Don't worry so much about the power/what gear you're in.



The percent change in rpm/tire size is the percent/change in gearing to compensate.

The diameter % change woks out to about the same as the RPM/effective gearing change...and going from 32's (Stock) to 33's is about a 3% change.

If the stock gears were 3.21, a 3.3 -ish gear would "get you back to stock" etc.


If the OEM tires were 29's with 3.21 gears...then you increased your final drive ratio by about 12% increasing to 33's, so, increasing a 3.21 by 12% gets you~ 3.6 gears...."to get back to stock" gearing.

If some of the effective final drive is from the rubber parts, and some is from the metal parts...it really doesn't matter.

As far as mpg goes....cruising in 5th at 60 mph at 2k on the tach, or cruising at 2k on the tach in 6th at 60 mph....is the same.



IE: Free gear swap = shift down....as far as your mpg goes...same thing. You end up with a final drive ratio....what ever that may be, due to the combination of metal and rubber "gears".



Considering the $ you'd spend to re-gear the diffs....do you realize the pay back period you'd need to break even on a difference in mpg?

Generally, if you do long drives on reasonably flat terrain....the larger tires act as a deeper over drive...but, have a higher rolling resistance due to the hysteresis of the taller sidewalls, etc.

The funny thing is that the vast majority of the time, MOST people who report a loss in mpg when they get new tires are driving faster than they did before the swap.

Like if you went from oem 29's to 33's, w/o recalibrating the speedometer and odometer....you're driving ~ 12% faster, and going ~ 12% further on the gas, than you thought.

So - lets say you thought you were going up a hill at 70 mph....you'd REALLY be going closer to 78-79 mph....if you used to have to down shift to 5th to maintain speed on that hill at 79 mph, but could do it in 6th at 70....with the 12% larger tires, you are downshifting to 5th just like you would have, BUT, THINK its to just go at the slower 70 mph speed....and think you lost a lot of power, etc.

And when filling up the tank...the odometer Says you went ~ 12% fewer miles, the little mpg gauge on the dash says you lost mpg, etc...all because of the human nature to drive the speedometer's speed, without slowing down/realizing the speedometer was off 12%, etc.



So - If the gears are going to cost ~ $750 per diff = $1,500.

At ~ 75 miles/day for work = ~ 20k miles /year.

As you're probably speeding, adding huge wind resistance, etc...you'd get more pay back, for free, by just driving ~ 12% slower than the speedo says....and adding 12% to the gauge's/filing station math, to get the true mpg.



Food for thought.

On the other hand - the lower the gears, the better off road...and, if you are going to one day mount 35's, etc...then, getting new lower gears will not save any money on gas...but, your clutch will thank you after a day on the rocks, etc.



BTW - all those gearing table clones assume a 1:1 final drive ratio...none of them have overdrive numbers, like our 6 speeds have....so they are all off by the difference between 1:1, and your actual 6th gear overdrive ratio.

IIRC, 6th is ~ 0.84, and 5th is 1:1

So shifting from 6th to 5th, is the same as dropping in 16% lower gears into the diffs, but staying in 6th....a new ~ 3.72 diff gearing equivalent.

~ = to the optional 3.76 gears you could have gotten in your diffs by checking a $50 option box.



IE: Re-gear your diffs from 3.21's to 3.72's by dropping from 6th to 5th...its free, and involves much less down time and money for the change, etc.

As far as your mpg goes...its the same thing....a particular rpm at a particular speed...that's all the mpg knows....and that's all the new diff gears do for you.



Want lower gearing than 3.72 diff equivalents, go crazy, and drop it into 4th!




IIRC, 4th is about a 1.25:1 ratio...so, another 25% deeper gear than 5th's 1:1

IE: If dropping to 5th got you the same as putting 3.72's in the diffs, shifting it into 4th would be like dropping in 4.65's into the diffs, closer to the 4.88's alot of people like.

And so forth.

Does this help get a feel for what the new gears will do for you mpg/acceleration wise?

Remember, most of the posts claiming you need new gears for minor tire changes are based on the driver looking at the wrong speedometer reading, and thinking, DAMN, I used to get up to speed SO much faster on the smaller tires!...I'd better re-gear!




(And I reiterate....the lower gears are much better OFF road, so, there's no universally right or wrong answer....it will be different for every driver, depending upon what they want to achieve.)

Good Luck!

 

how long is a clutch going to last taking off with 33s and 3.21s?
he knows he needs new gears he wants to know which ones
my personal opinion if i were he would be to uppgrade to the 44s from one of our sponsors, sell the parts out of his 30 44 axles and have 410s new brakes
and stronger axles.

 

Dayum...the guy's worried about $, and you want him to spend $3,000 on Rubi axles from Mopar, or whatever from the after market?



A coupla hundred back for the D30 helps, sure, but he's going to need installation, etc...big bucks for a guy worried about mpg/$.

He's read the swap threads I'm sure...I just wanted him to consider the equivalent outcome for alternatives if $ is tight.