I’m not posing the usual “should I get 4.56 or 4.88 gears for my 27” tires” and then have everyone get in a ****ing match of what I should get. I’ve got the formulas about tire diameter and rpm’s and gear ratios, so I can plug and chug all day trying different tire sizes and different gear ratios. How do I determine what is the optimum gear ratio for a given tire size. What is the optimum motor RPM that I should try to achieve? Is it something that is close to the stock tire size and gear ratio? What if I want a little more oomph for towing and off-road use. Or what if I want to be a little conservative for better gas mileage. I don’t want to know ‘what’, I want to understand ‘why’.
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Gearing theory
- Thread starter karstic
- Start date
yuppiexj
NAXJA Forum User
- Location
- fredericksburg VA
just under 3000 rpm @ 85 works for me.
I run 4.56s and 32s (31.6 now)
Going from 3.55 to 4.56 raised my mileage 3.5 points (yes it is a corrected speedo gear).
Driveability is much better, this is a 1988 4.0 aw4 with 206k on the clock.
Due to the increased rolling resistance, and aerodynamic drag of the lift and larger tires I found this to be a good range for me.
From my own experience trying to pull a hill at 65mph @ 1800 RPM in OD just wasn't happening, do not worry about spinning a 4.0 too fast (too many main bearings to worry about that.)
The closer to the torque peak you run, the more efficient the engine is.
(less gas pedal, fewer downshifts)
20.5 MPG last tank 40% highway
usually I get 19.5 - 20 (I really rag on it)
I hope that rant helps.
I run 4.56s and 32s (31.6 now)
Going from 3.55 to 4.56 raised my mileage 3.5 points (yes it is a corrected speedo gear).
Driveability is much better, this is a 1988 4.0 aw4 with 206k on the clock.
Due to the increased rolling resistance, and aerodynamic drag of the lift and larger tires I found this to be a good range for me.
From my own experience trying to pull a hill at 65mph @ 1800 RPM in OD just wasn't happening, do not worry about spinning a 4.0 too fast (too many main bearings to worry about that.)
The closer to the torque peak you run, the more efficient the engine is.
(less gas pedal, fewer downshifts)
20.5 MPG last tank 40% highway
usually I get 19.5 - 20 (I really rag on it)
I hope that rant helps.
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Okay, some theory... Larger tires are heavier and have greater rolling resistance. Because of that you will need to be turning a bit more than stock RPMs in order for it to FEEL like stock performance with larger tires. The larger the tires the more above stock RPMs you want it to turn. So, with 31" tires if you're turning 50 RPMs more than stock at 65 mph it'll feel pretty good. With 44" tires, however, you'd probably want at least a couple of hundred RPMs more than stock.
If you live in a mountainous area, or do mostly off-roading, or tow a heavy trailer, or just want more power off the line and aren't too worried about gas mileage at highway speeds, then you want the engine to turn more RPMs. If I did heavy towing I'd want at least 2500 RPMs at 65 mph.
If you live in Iowa, never tow, don't off-road much, do a lot of highway miles and want the best possible gas mileage on the highway, then you want the RPMs closer to stock, but probably not less than stock.
If you live in a mountainous area, or do mostly off-roading, or tow a heavy trailer, or just want more power off the line and aren't too worried about gas mileage at highway speeds, then you want the engine to turn more RPMs. If I did heavy towing I'd want at least 2500 RPMs at 65 mph.
If you live in Iowa, never tow, don't off-road much, do a lot of highway miles and want the best possible gas mileage on the highway, then you want the RPMs closer to stock, but probably not less than stock.
yuppiexj said:
Somewhat, it's good information, although I'm looking more for what factors come into play when choosing an axle ratio.
dmillion said:
Okay now we are getting somewhere. I realize with a heavier vehicle, larger tires, towing, etc. requires deeper gearing.
BillR
Toy Jeep driver!
- Location
- Flagstaff, AZ
All of that, plus a higher wind resistance due to being lifted.karstic said:
I run just over 3000 rpms at 80 mph. It feels good and it's still pretty smooth.
CW
NAXJA Forum User
- Location
- Coeur d'Alene, Idaho
The gearing is too high stock, the best gas mileage I ever got was when I dropped my 3.07's to 3.50's. This is with a standard but It probably applies to the autos too. I know a lot of people that have old amc 258ci's and no overdrive that get equal or better gas mileage than I did with a stock 4.0L. They are just turning a higher more efficient rpm that puts them closer to peak power. RPMs don't nescessarily effect gas mileage, it's how far you have to push the skinny peddle to achieve a certain speed or rate of acceleration.
old_man
NAXJA Forum User
- Location
- Loveland, Colorado
You really need to plot your engine torque/horsepower curve against your driving needs. Having a too tall gear simply means that the engine isn't operating at an efficient RPM with your gears and as such it can't generate the power required. If the RPM's are too low you simply bog the engine. If they are too high, you have no top speed and you end up using too much gas and putting an excessive strain on the engine.
CW
NAXJA Forum User
- Location
- Coeur d'Alene, Idaho
http://www.picturevillage.com/photo/showpic.php?aid=11183&pid=236099&uuid=3022
The desktop dyno2000 files are available at http://autos.groups.yahoo.com/group/XJOG/files/
incase you want to add mods to the figures.
The desktop dyno2000 files are available at http://autos.groups.yahoo.com/group/XJOG/files/
incase you want to add mods to the figures.
Yucca-Man
NAXJA Member # 683
- Location
- Castle Rock, CO
No way...no way would I do 5500rpm in a 4.0L. How about any factory-supplied charts instead? That one just doesn't sound right.CW said:
CW
NAXJA Forum User
- Location
- Coeur d'Alene, Idaho
That is taken from my dyno2000 program and it matches the factory hp/tq numbers, peak hp is at 5000 rpms just below the rev limiter. They are a computer simulation so take it for that, but I entered in the factory specs for the program with some help from Dr. Dyno.Yucca-Man said:
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"The gearing is too high stock"
I agree. That is why I switched to 3.55 gears in my manual-tranny '01, even though it is my wife's daily driver and hence will never have taller tires than the 235/75x15s that are on it now. I also live in Colorado and got tired of having to go down to third gear all the time on the Interstate!
I agree. That is why I switched to 3.55 gears in my manual-tranny '01, even though it is my wife's daily driver and hence will never have taller tires than the 235/75x15s that are on it now. I also live in Colorado and got tired of having to go down to third gear all the time on the Interstate!
Blackash132
NAXJA Forum User
- Location
- St. Louis, MO
OK so if I want around 2600RPM's at 80mph with 33's and a D44 I need 4.56 or 4.88 gears?
Also if everyone hates the D30 in the front what do you swap in its place? another D44 or is there a better option?
I simply want to be able to drive nicely on the highway but not have to re do the jeep when I get more trail savvy and want to go bigger... I think 33's and a RE4.5 will give me the room to "grow" but still be a managable DD.
Also if everyone hates the D30 in the front what do you swap in its place? another D44 or is there a better option?
I simply want to be able to drive nicely on the highway but not have to re do the jeep when I get more trail savvy and want to go bigger... I think 33's and a RE4.5 will give me the room to "grow" but still be a managable DD.
- Location
- Terra Firma
Yoiu really don't want 2600 RPM at 80 MPH. That's not enough gear by far. What gear are you going to have to be in when you're in a 55 MPH zone -- or a 35 MPH zone?
Theory is that, ignoring for the moment air stream resistance which increases exponentially with speed, the most efficient cruising speed is the engine speed where the maximum torque occurs. With the 4.0L, at least according to the factory that¿'s different depending on year. The Renix torque peak was 2400 or 2500 RPM depending on which catalog you look at. The first HOs had a torque peak considerably higher, then they dropped it back and in 1999 the peak was at 3000 RPM. The 1999 Jeep catalog shows the torque curve for the 4.0L and it is almost flat from 215 ft-lbs at 1200 RPM all the way up to 4500 RPM, where it falls off steeply. The peak at 3000 RPM is not a typical high point or apex of a smooth parabolic curve, it is actually a minor spike at that point.
However, I agree completely that the factory gearing isn't deep enough. I go back to when AMC first introduced this line of engines. This was before we had overdrive transmissions. Back then the manuals got MORE gear than the automatics, not less. Typically, stock AMCs came geared to run 24 MPH per 1000 RPM. That's 2500 RPM at 60 MPH. In that configuration, my 1966 Rambler American routinely delivered 28 MPG on the highway and 22 MPG around town, and was still capable of beating the vaunted Mopar slant 6 in a drag. And the Rambler American was about as aerodynamic as a Cherokee.
That ratio meant cruising at 3000 RPM to go 72 MPH. At that point gas mileage began to drop because the engine was beyond the torque peak, and also the aerodymanic drag was building up.
A current stock XJ with automatic turns about 2200 to 2300 RPM at 70 MPH. That's better than the 3.07 ratio for the 5-speeds, but IMHO it still isn't enough. If 2500 RPM at 60 MPH was good enough for the AMC engineers back when they developed the engine, it will still work. However, 80 MPH would be nearly 4000 RPM and I think that's pushing it a bit for engine longevity. If you actually spend a significant amount of time cruising at 80 MPH, I think you should be looking to cruise at between 3000 and 3500 RPM.
Theory is that, ignoring for the moment air stream resistance which increases exponentially with speed, the most efficient cruising speed is the engine speed where the maximum torque occurs. With the 4.0L, at least according to the factory that¿'s different depending on year. The Renix torque peak was 2400 or 2500 RPM depending on which catalog you look at. The first HOs had a torque peak considerably higher, then they dropped it back and in 1999 the peak was at 3000 RPM. The 1999 Jeep catalog shows the torque curve for the 4.0L and it is almost flat from 215 ft-lbs at 1200 RPM all the way up to 4500 RPM, where it falls off steeply. The peak at 3000 RPM is not a typical high point or apex of a smooth parabolic curve, it is actually a minor spike at that point.
However, I agree completely that the factory gearing isn't deep enough. I go back to when AMC first introduced this line of engines. This was before we had overdrive transmissions. Back then the manuals got MORE gear than the automatics, not less. Typically, stock AMCs came geared to run 24 MPH per 1000 RPM. That's 2500 RPM at 60 MPH. In that configuration, my 1966 Rambler American routinely delivered 28 MPG on the highway and 22 MPG around town, and was still capable of beating the vaunted Mopar slant 6 in a drag. And the Rambler American was about as aerodynamic as a Cherokee.
That ratio meant cruising at 3000 RPM to go 72 MPH. At that point gas mileage began to drop because the engine was beyond the torque peak, and also the aerodymanic drag was building up.
A current stock XJ with automatic turns about 2200 to 2300 RPM at 70 MPH. That's better than the 3.07 ratio for the 5-speeds, but IMHO it still isn't enough. If 2500 RPM at 60 MPH was good enough for the AMC engineers back when they developed the engine, it will still work. However, 80 MPH would be nearly 4000 RPM and I think that's pushing it a bit for engine longevity. If you actually spend a significant amount of time cruising at 80 MPH, I think you should be looking to cruise at between 3000 and 3500 RPM.
joecool
NAXJA Forum User
- Location
- Akron, Ohio
I dont know if this is what you were originally looking for, but I found this chart to be very helpful. It shows tire size vs. gear ratio, and highlights the areas for best fuel economy, for optimal performance, and for more power. It also shows rpms. http://rough.roughcountry.com/Gear Ratio Guide.htm
CW
NAXJA Forum User
- Location
- Coeur d'Alene, Idaho
Engine RPM=Gear Ratio x Trans final drive x mph x 336/tire diameter
Ax-15 (.79 OD)
4.88's = 3140 rpms
4.56's = 2934 rpms
4.10's = 2638 rpms
AW4 (.69 OD)
4.88's = 2742 rpms
4.56's = 2562 rpms
4.10's = 2304 rpms
I agree with eagle, I would want to be around 3000rpms at 80. That chart assumes a 1:1 ratio so you have to do the math anyway to figure out what rpms you will be turning.
Ax-15 (.79 OD)
4.88's = 3140 rpms
4.56's = 2934 rpms
4.10's = 2638 rpms
AW4 (.69 OD)
4.88's = 2742 rpms
4.56's = 2562 rpms
4.10's = 2304 rpms
I agree with eagle, I would want to be around 3000rpms at 80. That chart assumes a 1:1 ratio so you have to do the math anyway to figure out what rpms you will be turning.
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Blackash132
NAXJA Forum User
- Location
- St. Louis, MO
Thanks guys that chart and equations realy helped! I have been looking for a rough chart like that ... I appreciate it!
