First off i know this horse has been beat to death and I've read a ton of threads on it. But I see a 50/50 split on this combo. I see people saying I have it and love it and others saying its way over geared. People say if you are towing have a lot of hills then go for it, obviously if you are going bigger tires then go for it. If you drive a lot of highway then get 4.10, you will be revving up to high with 4.56, then people say 4.0 are fine revving up high on highway. People are saying a ton of different rpm numbers driving at certain speeds. So I have a 2000 cherokee 4.0 auto with 3.55 lifted 4" on 31 duratracs and I'm never going bigger tires. I'm using the rig for hunting, so I drive at most 2 hours to hunting spots. I also use it for light wheeling, but not much. To hunting I sometimes go over the willamette pass, other spot is flat highway until hitting hunting hills. I want to know people who have my set up (4.0 auto 31's) with 4.56 gears and what Rpms are they at going 60 and going 70. I would be happy to get 4.56 gears if I know at like 65 I would be at 2500 Rpms. Thanks in advance
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4.56 on 31's
- Thread starter Levathain
- Start date
x-jay vision
NAXJA Forum User
- Location
- Albuquerque, NM
I dont really stare at my Tach while driving, so I don't know where I am at RPM-wise, but the motor sounds nice, and when I hit the gas there is more power to give. If I let off the gas at 65 it doesnt slow down rapidly, so I know I'm running right where the engine wants to be. I average 13.6 mpg with all my driving, includes about 70% around town and 30% highway. 2500 RPM at 65 seems like sort of an arbitrary number, why does it matter?
- Location
- Hammertown, USA
One of our locals ran 4.56 and stock tires for about a year. He got 16-17 mpg and kept up with me on the highway.
I wouldnt hesitate.
I wouldnt hesitate.
- Location
- Hammertown, USA
Decide for yourself.
4.56, 31's.
4.56, 31's.
- Location
- Hammertown, USA
For what its worth, on the rare occasion I drive my rig on the highway, I'm at 2700 to 3200 rpm's.
The jeep DOES NOT CARE. It doesnt even really hinder gas mileage, since you're spinning fast but not deep in the throttle.
The jeep DOES NOT CARE. It doesnt even really hinder gas mileage, since you're spinning fast but not deep in the throttle.
- Location
- Wasatch Range
Great point, Cal. Cruising RPM is less of a factor, with the amount of throttle ( = fuel) required to maintain said RPM being much more important. This is why folks running 33's with 3:73 gears will burn more fuel to maintain 65 mph than the folks running 4:88 gears.
- Location
- Hammertown, USA
To the OP:
Throw it in third and go for a drive. 3.55 and 31's in third will be slightly higher RPM's than 4.56 and 31's in 4th. Real world test for the win.
4.56 on left, 3.55 on right
Throw it in third and go for a drive. 3.55 and 31's in third will be slightly higher RPM's than 4.56 and 31's in 4th. Real world test for the win.
4.56 on left, 3.55 on right
Vanimal
NAXJA Forum User
- Location
- escondido, ca
i run 3100rpm's on the highway at 80 (37's/5.38's) and the noise drives me nuts. it does cruise pretty good at that rpm and mileage isnt bad, all things considered.... it's just tiresome on long drives
WB9YZU
NAXJA Forum User
- Location
- Madison, WI
There is something flawed with the thought process here.
A stock 4.0L XJ with Stock tires gets 20-24mpg HWY depending on whether it's auto or manual.
That 16-17 mpg (assuming the figures were adjusted for the speedo being way off), is a serious drop in efficiency (about 20%).
Exactly how you folks figure that RPM doesn't matter is beyond me at the moment. Engines are simply air pumps. The faster they turn, the more air they flow, even with the throttle opened the same amount.
Who says you are deeper in the throttle with 4.56's vs stock gears in cruise? Did anyone read the TPS value when they took their "impression"? How deep you are in the throttle at cruise depends on a number of factors.
Tire size, tire inflation, wind, levelness of th asphalt, load, the nut behind the wheel, and what gear you are in.
If you want to test that out with a stock 4.0L XJ, just leave it in 3 when you get on the HWY. The TC will still lock, but won't enter OD.
Take the same exact route, driving the same exact way, with the same load, in both OD and 3rd.
YMMV, but the result won't. You will burn more fuel over the same distance at the same speed.
There is generally an bell curve to milage., That's why Honda added an extra gear to the 4spd Civics. They got Reasonable mileage around town, good gas mileage at 55, but on the Interstate, that little 4 popper was out of it's power band and the mileage suffered.
I have seen this curve on every vehicle I have ever driven, including XJs. There is always a sweet spot where gearing, load, and the torque curve all come together.
- Location
- Hammertown, USA
Maybe your XJ got 20-24 stock. Rating for a 1987 Jeep Cherokee (which this one was) is 16 highway, 18 combined, 21 stock.
And you're right, an engine is basically an air pump that pumps more air at higher RPM's. And with a carburated car, that would directly translate to MPG as that air sucked fuel in with it.
That said, on our EFI 4.0 jeeps, fuel use is directly related to throttle position and MAP - RPM's are not a primary factor in fuel load.
cpefy3
NAXJA Forum User
- Location
- Virgin Islands
If you never plan on going bigger, in regards to tire size, I would chose 4.10.
- Location
- In the islands
This have always been an interesting discussion for me, especially the part that says the 4.0 love high RPM's and achieves better gas mileage. Both my previous 1994 and now my 1996 XJ give the greatest returns for my gas money below 3,000 RPM. The 1994 was all OEM specs except that it ran 235 X 75 X 15 tires. It got 20+ MPG all day on the highway. The1996 did that until I changed tire size to 30 x 9.5 X 15 tires. Then the MPG went down to about 17 and got worse in the hill country because it was always in 3rd gear. Today, the XJ is turning some 80% worn 32 X 12.5 X 15 tires through 4.10 gears. I did a calculation today after 3 tank full driving about 80% highway and 20% city street here in North East Florida and got 20.25 MPG. My sustained speed is about 2,000 RPM at 65 MPH +/-.
What I trying to say is, I may have gotten the only two XJ,s / 4.0's that prefer to remain below 3,000 RPM. NO, I am not doubting anyone result it is just that my results have consistently been different.
My opinion for the OP, The gearing may be OK for low offroad speed or hill climbing but may not be very good on long highway runs.
What I trying to say is, I may have gotten the only two XJ,s / 4.0's that prefer to remain below 3,000 RPM. NO, I am not doubting anyone result it is just that my results have consistently been different.
My opinion for the OP, The gearing may be OK for low offroad speed or hill climbing but may not be very good on long highway runs.
WB9YZU
NAXJA Forum User
- Location
- Madison, WI
Well, let's see now...
A Carburetor works by flow. The more flow, the more fuel. Where you don't have RPM, you need to have manifold vacuum to create the difference in pressure across the carb and generate airflow to pull fuel from the jet ports. There is a separate set of ports for idle, and for off-idle operation. There is also a acceleration pump to add extra fuel when you transition from a lower RPM to a higher one to increase throttle response.
EFI works the same way, except that the computer calculates air flow based on MAP and RPM and delivers the amount of fuel based on a lookup table directly to the intake valve.
Between the two systems, there is no effective difference in operation:
More Flow = More Fuel
Less Flow = Less Fuel
Most 4 cycle engines with stock manifolds and stock cams generate the most torque on the lower end of the RPM range, and the most HP towards the top of it. The 4.0L has a fairly flat torque curve, but the HP is more slanted and the curves cross at some point.
Now this is where I start thinking out loud...
It seems to me that if you want fuel efficiency at Cruise, you find out what that point is, and set your gears for that RPM. Since the engine is most efficient at that point, you will not need to add as much fuel to achieve the power required for cruise.
The clearest power curve I could find was this one which was an attachment on JF.
So according to the graph, the engine will be most efficient at 2.6K or so, right?
The AW4 has a 1:1 3rd gear an a .75 OD gear ratio.
If you use this calc, you can determine the best gear to match your tire size, just remember to clear out the RPM line each time.
http://www.4lo.com/calc/gearratio.php
So as an example: 33" tire (We all know that's more like a 30.5") and a 4.56 gear with an Automatic (.75od) and the TC in 2hi (1:1).
Hit Calculate and we see we get 2264.
Running the numbers, we really need a 5.13 gear with 33's to get to the crossover point.
From experience is anyone getting 19 mpg with 33's and 5.13's?
Now this is the interesting part to me anyway...
From the factory, if all my thoughts on engine efficiency are true, then a Stock XJ with 3.55 gears, AW4, and 27" tires came with the wrong gears for the engine curve, infact, way off.
Where does that put the more fuel efficient Manual XJ with 3.07 Gears?
A AX-15 has a OD of .79, and though the calc we get 1962 rpm. That's way below the crossover point. Lugging down the road it appears.
It must only take about 50 HP to keep a Stock XJ rolling down the road @65. Somehow, that seems a bit low. Add a lift and huge tires, and I don't think that 50HP is reasonable at all.
A Carburetor works by flow. The more flow, the more fuel. Where you don't have RPM, you need to have manifold vacuum to create the difference in pressure across the carb and generate airflow to pull fuel from the jet ports. There is a separate set of ports for idle, and for off-idle operation. There is also a acceleration pump to add extra fuel when you transition from a lower RPM to a higher one to increase throttle response.
EFI works the same way, except that the computer calculates air flow based on MAP and RPM and delivers the amount of fuel based on a lookup table directly to the intake valve.
Between the two systems, there is no effective difference in operation:
More Flow = More Fuel
Less Flow = Less Fuel
Most 4 cycle engines with stock manifolds and stock cams generate the most torque on the lower end of the RPM range, and the most HP towards the top of it. The 4.0L has a fairly flat torque curve, but the HP is more slanted and the curves cross at some point.
Now this is where I start thinking out loud...
It seems to me that if you want fuel efficiency at Cruise, you find out what that point is, and set your gears for that RPM. Since the engine is most efficient at that point, you will not need to add as much fuel to achieve the power required for cruise.
The clearest power curve I could find was this one which was an attachment on JF.
So according to the graph, the engine will be most efficient at 2.6K or so, right?
The AW4 has a 1:1 3rd gear an a .75 OD gear ratio.
If you use this calc, you can determine the best gear to match your tire size, just remember to clear out the RPM line each time.
http://www.4lo.com/calc/gearratio.php
So as an example: 33" tire (We all know that's more like a 30.5") and a 4.56 gear with an Automatic (.75od) and the TC in 2hi (1:1).
Hit Calculate and we see we get 2264.
Running the numbers, we really need a 5.13 gear with 33's to get to the crossover point.
From experience is anyone getting 19 mpg with 33's and 5.13's?
Now this is the interesting part to me anyway...
From the factory, if all my thoughts on engine efficiency are true, then a Stock XJ with 3.55 gears, AW4, and 27" tires came with the wrong gears for the engine curve, infact, way off.
Where does that put the more fuel efficient Manual XJ with 3.07 Gears?
A AX-15 has a OD of .79, and though the calc we get 1962 rpm. That's way below the crossover point. Lugging down the road it appears.
It must only take about 50 HP to keep a Stock XJ rolling down the road @65. Somehow, that seems a bit low. Add a lift and huge tires, and I don't think that 50HP is reasonable at all.
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- Location
- Hammertown, USA
:huh:
Sure thing Ron!
Sure thing Ron!
To add to the confusion. From my seat of the pants feel, no real data.
I am running 4.56 with 33s. Feels great. With the new stroker, I can get 17 MPG real world on the highway, loaded down with who knows how much weight.
If you really, truly, absolutely never are going above 31s (I don't believe it, frankly), then go with 4.10s. Otherwise, do the gearing to 4.56 the first time, and not have to spend the money twice. The real world difference between 4.10 and 4.56 is not that much, and any gas savings you might get will go out the window if you have to regear later.
I can do 70-75 mph comfortably with Gomer for 200 miles. And I have one of the noisiest fans in the universe. If you drive 60-65 mph, I don't see an issue with 4.56 on 31s. If you like to drive 80 mph, that's a different story. Pulling a trailer with 4.56 will be a nicer experience.
Light throttle is more important that RPM. My mileage improved after the stroker installation; my gear ratios didn't change. I am now much less into the throttle for the same speed than before.
David Bricker / SYR
I am running 4.56 with 33s. Feels great. With the new stroker, I can get 17 MPG real world on the highway, loaded down with who knows how much weight.
If you really, truly, absolutely never are going above 31s (I don't believe it, frankly), then go with 4.10s. Otherwise, do the gearing to 4.56 the first time, and not have to spend the money twice. The real world difference between 4.10 and 4.56 is not that much, and any gas savings you might get will go out the window if you have to regear later.
I can do 70-75 mph comfortably with Gomer for 200 miles. And I have one of the noisiest fans in the universe. If you drive 60-65 mph, I don't see an issue with 4.56 on 31s. If you like to drive 80 mph, that's a different story. Pulling a trailer with 4.56 will be a nicer experience.
Light throttle is more important that RPM. My mileage improved after the stroker installation; my gear ratios didn't change. I am now much less into the throttle for the same speed than before.
David Bricker / SYR
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- Location
- Torrance, CA
Ok, let me break it down for you Ron, in simple terms that you can grasp.
So an engine is essentially a giant air pump, the 4.0 happens to displace 4.0 liters of air. Now for argument's sake lets say for every thermodynamic cycle of the 4.0 motor, it needs 4.0 liters of air, and with that air you have 1 part fuel for 14.7 parts air. So lets say your 4.0 being 100% efficient and have no issues moving air is turning at 1500 rpms. Every minute your 4.0 motor's turning those rpms it will need 750 liters of air and 51 liters of fuel. Now lets say you spin our hypothetical 4.0 engine up to 2500 rpms. It's now going to need 1250 liters or air a minute and 85 liters of fuel a minute. So going off of your graph, the most efficient cruise rpm would be the lowest RPM where the motor makes enough torque to maintain speed. Which looking at your graph would be around 1600 rpms
Now lets say you add big tires are increase the amount of torque required to maintain speed, you are going to need to raise the cruising rpm to maintain those speeds, but you are going to want to keep the rpms the lowest to maintain the best MPG.
This is why OEMs will put such low gears in big trucks, for example a V10 F350 tows great with 4.30 gears, but it gets single digit MPG numbers. Most of the trucks came with 3.73 gears to lower the cruise rpms for better MPG.
So an engine is essentially a giant air pump, the 4.0 happens to displace 4.0 liters of air. Now for argument's sake lets say for every thermodynamic cycle of the 4.0 motor, it needs 4.0 liters of air, and with that air you have 1 part fuel for 14.7 parts air. So lets say your 4.0 being 100% efficient and have no issues moving air is turning at 1500 rpms. Every minute your 4.0 motor's turning those rpms it will need 750 liters of air and 51 liters of fuel. Now lets say you spin our hypothetical 4.0 engine up to 2500 rpms. It's now going to need 1250 liters or air a minute and 85 liters of fuel a minute. So going off of your graph, the most efficient cruise rpm would be the lowest RPM where the motor makes enough torque to maintain speed. Which looking at your graph would be around 1600 rpms
Now lets say you add big tires are increase the amount of torque required to maintain speed, you are going to need to raise the cruising rpm to maintain those speeds, but you are going to want to keep the rpms the lowest to maintain the best MPG.
This is why OEMs will put such low gears in big trucks, for example a V10 F350 tows great with 4.30 gears, but it gets single digit MPG numbers. Most of the trucks came with 3.73 gears to lower the cruise rpms for better MPG.
- Location
- Lakeside, CA
Where did you find that graph? Torque and HP are crossing at the wrong point, they always intersect at 5252rpm. HP = (RPM* TRQ)/5252. 4.56's are IMHO the best choice for 31-33" tires.
WB9YZU
NAXJA Forum User
- Location
- Madison, WI
Mark, I noted where I got it; JF. It's not an unusual graph. Here's another:
http://www.allpar.com/images/engines/power-318.gif
The graph simply shows the relationship between Torque and HP, and obviously the scales are adjusted so they overlap.
Sideways, I understand why OEM uses numerically low gears in some vehicles (Mostly larger CI, low end torque engines). However, if you looked at a 2.5L or just about anything Japanese, you see a different relationship. Those engines don't make low end torque, so to get into a range where the engine creates power, the gear ratios increase substantially.
The problem with the WOT air pump model is that it doesn't model the actual airflow needed to maintain any given RPM for a given engine load. So the closest thing I have to look at is the engine curve. Do you have a better model?
I understand the relationship between keeping your RPMs low and gas mileage (See post #11). Why, if the engine is more efficient at a different RPM, would the ratio be lower was my pondering.
Seems to me after typing all of this, is that it is a matter of balance; the balance of power required to maintain cruise, and the RPM which generates the minimal output required to do so.
As you say, larger tires etc, moves that balance point further up the curve, which requires deeper gears to maintain balance. Makes sense to me.
Again, If you have a magic calculator to find that balance, roll it out buddy
http://www.allpar.com/images/engines/power-318.gif
The graph simply shows the relationship between Torque and HP, and obviously the scales are adjusted so they overlap.
Sideways, I understand why OEM uses numerically low gears in some vehicles (Mostly larger CI, low end torque engines). However, if you looked at a 2.5L or just about anything Japanese, you see a different relationship. Those engines don't make low end torque, so to get into a range where the engine creates power, the gear ratios increase substantially.
The problem with the WOT air pump model is that it doesn't model the actual airflow needed to maintain any given RPM for a given engine load. So the closest thing I have to look at is the engine curve. Do you have a better model?
I understand the relationship between keeping your RPMs low and gas mileage (See post #11). Why, if the engine is more efficient at a different RPM, would the ratio be lower was my pondering.
Seems to me after typing all of this, is that it is a matter of balance; the balance of power required to maintain cruise, and the RPM which generates the minimal output required to do so.
As you say, larger tires etc, moves that balance point further up the curve, which requires deeper gears to maintain balance. Makes sense to me.
Again, If you have a magic calculator to find that balance, roll it out buddy
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