31" on 4.56

[winkosmosis]

It's physics (which i won't pretend to have a firm grasp of). Hold a 5 lb weight in your hand with your elbow bent 90*. Now extend it out so your arm is straight. The weight is the same, but the leverage exerted by the longer lever causes more strain to support the same weight. Now imagine doing that but increasing the weight when you increase the length of the lever (your arm). It's gonna feel like a LOT more weight. Tests have been done involving sports cars where they run the same overall height tire but change the wheels out. Either plus sizing or just experimenting with lither and heavier wheels. The heavier the wheel and tire combo was, the slower the car got, by a higher degree than would have been predicted by just adding the weight to the car.

EDIT: I actually had the benefit of going from 30" tires to 33" tires without changing the lift. Obviously, with the stock gears, it was a dog. But when I regeared to 4.88s, it did indeed improve by quite a bit, but still considerably slower than when I was on 30's with stock gears and steel wheels, which was somewhat slower than when it was stock all around.

What I'm arguing is that the significance of the bigger tires is in the leverage from the bigger tire, which is why you regear, but not so much from the weight. So to me the best thing to do is regear so you maintain a stock ratio. For 33" tires I think that would be 4.11

 

[jth877]

Has nothing to do with the weight of the tire/wheel, but the acceleration of mass. It takes more energy to accelerate a larger mass to certain speed than is does a small mass. Liner or centripetal. BUT, with a larger mass, the object takes more force to stop, also meaning that it take a longer time for the tire/wheel to stop spinning with the same braking force put upon it. So, it would depend on how much stop and go traffic you drive in. The only benefit to a larger rotating mass is that it "wants" to keep going once it starts to move.

They should partially cancel each other out, but you are not going to get back the extra energy put into moving a bigger tire because of the friction occurred with a wider tire/ larger mass. I also noticed that shops don't balance larger tires that well- that would kill some mileage (and bearings) as well.

 

[89xj]

What I'm arguing is that the significance of the bigger tires is in the leverage from the bigger tire, which is why you regear, but not so much from the weight. So to me the best thing to do is regear so you maintain a stock ratio. For 33" tires I think that would be 4.11

yes, the increase in leverage or tire size is the main reason, but a 33" tire weighs more then a 28" tire. add that weight, resistance, weight of bumper/skids/lift/winch, camping gear..... and now it takes more effort to move the load.

gearing lowerer(deeper) then stock cancels out the increased effort it takes to move the increase in load.

now if everything about your xj stays the same, other then the diameter of the tire, then gear to match the stock set up.

 

[Darky]

What I'm arguing is that the significance of the bigger tires is in the leverage from the bigger tire, which is why you regear, but not so much from the weight. So to me the best thing to do is regear so you maintain a stock ratio. For 33" tires I think that would be 4.11

We regear because the tire diameter is part of the overall gear ratio. That's what the calculators are figuring out when they talk about getting back to a stock ratio. 3.55:1 running through a 28" tire is a much lower gear ratio than 3.55:1 running through a 33" tire. The tire is essentially another "cog" in the chain.

 

[winkosmosis]

We regear because the tire diameter is part of the overall gear ratio. That's what the calculators are figuring out when they talk about getting back to a stock ratio. 3.55:1 running through a 28" tire is a much lower gear ratio than 3.55:1 running through a 33" tire. The tire is essentially another "cog" in the chain.

I know, that's what I'm saying. Other people are arguing that you need to regear even deeper than that for the extra rotating mass and weight of your gear.

Well the weight of your gear would matter a lot, but how often are you carrying all that? Once a week at the most? The rest of the time you're reducing your gas mileage.

The weight of skid plates isn't significant. My full width belly skid is about 50lb, and the front skid is maybe 30lb. Assuming a gas tank skid is another 50lb, you're at 130lb, the weight of a passenger... About 5% the total weight of the XJ

 

[ktm racer 419]

you can play with your brainy physic math all you want, i'm not readin into all that

but what i have learned from experience: GEARZ IS GOOD

i run 5.38's with 35's
4.0/aw4/231/d44/d60 with cheap oil in each if anyone cares.

here is the breakdown:

i average 15 mpg city (heavyfoot) 4000 rpm's regularly

cruising around 55 i get low to mid 20's depending on how the moons aligned that day

highway around 70-75 i average around 18

and 75+ it all goes down the tube. maybe 11-12, but why do i need to drive my jeep much faster than that?

i hit the governor at 91 mph.

my take on it is this:

more gearing= less shifting on hills= less downshifting, and less slipping of the torque converter= more milage

the engine almost never "lugs". almost always right in the beginning of the sweet spot the 4.0 loves to make power and runs the most efficient.

but hey i dont know math and i'm only speaking from experience, what do i know?:confused1

 

[Lightfoot]

Redid that chart into something a bit more useful to outsiders (not me). It does all the math for you, just input your jeep information.
This was the best way that I know to share it.
http://spreadsheets.google.com/ccc?key=0AuZqE-GkWkJ0dHdWQl8tcFh2RzRWWnZlNWtBZ1Q2bVE&hl=en
The file will open, but not be completely readable. Go to file on the spreadsheet page, and go to download as. select Excel, and open it. It should be readable.

 

[Mike H.]

I ran 31's with 4.56's for a year and did a 440mile trip at 70-75mph (2800-3000rpms) weekly.
Vehicle routinely knocked down 18+ mpg.
I now run 33's on the 4.56's run 2600-2800 rpms (440mile trip at 70-75mph ) and knock down... 18+ mpg, and if I get a tail wind for 3-4 hours straight 20-21mpgs.

 

[winkosmosis]

Are you guys with the deeper gears factoring odo error into your mileage calculations? You really drove a lot shorter distance than your speedo says

 

[garr]

Are you guys with the deeper gears factoring odo error into your mileage calculations? You really drove a lot shorter distance than your speedo says

+1, it's either lack of a recalculated speedo or some of these guys with lifted, big tired, overweight, low geared Bricks on wheels are defying the laws of physics! :eeks1:

Some of these guys are getting better numbers than I was getting with my factory (Stock) new 4 Banger equipped early 80s XJ.

 

[Darky]

Are you guys with the deeper gears factoring odo error into your mileage calculations? You really drove a lot shorter distance than your speedo says

I'm pretty sure ktm understands about odo correction...

And I'm not referring to vehicle weight. Yes, vehicle plays a role, but let's say going from stock to 33s adds 100lbs to the vehicle. You're adding it at the worst possible place: unsprung rotating mass. That's where it'll affect the most areas of vehicle performance the worst.

 

[1990heepxj]

im seeing it as the same thing. noone is factoring in the fact theyre doin alot less miles then the odo will read. everyone is saying they get 18 so stock gears stock tires performance wise. and ur also saying stock setup is for fuel efficiency. so wouldnt u think they would gear it for a bit more power if it got the same milage no matter the gears. so ya if ur on 31s and 4.56s theres no way in hell ur getting 18 mpg highway its not physically possible since it was stated the over gearing means ur dumping fuel efficiency for power. the combination of responses on here is basically backing that its dumb to over gear an outrageous amount. my buddies xj has an ax-15 manual 33s and 4.10s he can chirp tires shifting up to 3rd with minor rpms. if i were the OP id put the 4.56s if he has an auto and deal with the over deep gears til i get my 33s. but look if ur on 31s and 4.56s and u have an auto ur d shaft is turning .83 of a turn more then it would stock so ur basically going about 20% fewer miles then ur odo will show due to overgearing

if ur on 33s u need 4.10s on a manual and 4.56s on an auto if u want plenty of power and not too much mpg loss. what my point is yes i am all for over gearing to compensate for the extra few hundred pounds of added vehicle weight/gear but im against over gearing to the extent of say 4.56s/31s or 4.88s/33s because no matter which trans u have ur way too deep for a dd, but awesome for a dedicated offroader.

my one good example for this is my yota crawler on 5.29s and 37s could chirp second on a bone stock 22re and the stock gearing was 4.10s with 31s. i coulda had 4.88s and been exactly stock but i had 5.29s and had a ton of power even on a 4 banger with only one step past the necessary ratio. so if ur on 4.88s and 33s ur a full 2 ratio steps higher then necessary

 

[JAS]

Being in a 97xj on 31x10.50 and 4.88 with AW4

I have re calibrated the speed. I speedo says 72-73 MPH and the GPS says 70.

I am sorry to say that I do not have an accurate MPG reading for now. but I do not believe that I am over geared.

The power around town is great. It no longer shifts on freeway uphills. Yes.. I now longer run 75+MPH but then it is a lifted Jeep and should not be running that fast. I take my other cars when I feel the need for speed.

 

[Ivan]

can i say ur all nuts with ur over gearing way past stock performance. use http://www.therangerstation.com/tech_library/Gear_Tire_RatioChange.html. to the OP if u drive a manual ur going to be massively over geared at 4.56s and 31s if ur an auto u wont be quite as bad but still over geared.

I dunno... On my 5 spd I'm running 32's with 4.56's, which isn't that much of leap above 31's. It runs just fine at highway speeds. I live in a hilly area, and going up hills in 5th gear is pretty sweet too. I have no regrets....

 

[Rod Knee]

so ur basically going about 20% fewer miles then ur odo will show due to overgearing

I'm on 31's and stock gears thinking about regearing also. I'm leaning toward going to 4.56 because I envision towing in my future and I'd rather err in going too low (num. high). Apart from my concerns about towing/hauling a heavy load (especially at high elevations), 31's on stock gears isn't much of an issue.

Regearing is expensive and I always hesitate to make unnecessary, expensive modifications that can't be recovered through insurance in the event the vehicle is totaled. I'm considering just having a second set of stock sized tires for towing as an alternative to regearing. Even if I include my four 31s and wheels as cargo when towing with the stock tires, they only amount to about the weight of one large or two small passengers.

 

[90 waggy]

find a 4 banger auto cherokee or wrangler and theres your 4.56 gears already set up. might have to do some bearings....cheap right?

 

[srmitchell]

Im on 3.55's with 31's. I can get 20 freeway, but add ANY heill, and I have to down shift, and I get about 15. It sucks.

I'm going to 4.56 in 2 weeks, I'll let you know. 32's within 2 months.

 

[Rod Knee]

Actually, I think the 4 cylinder engines had 4.10's. They aren't that easy to find around where I live, granted I haven't hit any pick/pulls yet.

 

[winkosmosis]

Being in a 97xj on 31x10.50 and 4.88 with AW4

I have re calibrated the speed. I speedo says 72-73 MPH and the GPS says 70.

I am sorry to say that I do not have an accurate MPG reading for now. but I do not believe that I am over geared.

The power around town is great. It no longer shifts on freeway uphills. Yes.. I now longer run 75+MPH but then it is a lifted Jeep and should not be running that fast. I take my other cars when I feel the need for speed.

I can drive 75mph all day long with 31" tires and 2" lift. Your problem is being overgeared

 

[90 waggy]

i musta found a rare one with the tow/offroad package. i was wondering why it had skid plates. :dunno: