please explain RPM's/gas consumption/gearing

Hey guys,

Can any of you link me to or explain the relationship between all of these? It is correct that your engine has to work harder to turn say, 33's with 3.55s, at 2500 rpms than the same 33s with 4.56s at the same rpms isn't it? Also at the same 2500 rpms with 4.56s you will be using less gas than 2500 rpms with 3.55s?

I got into a discussion/argument with my father and he thinks it is rediculous to think I will not see an increase in rpms and a reduction in my gas mileage if I regear from 3.55s to 4.56s. I told him I drive my jeep in 3rd gear right now, running at 2500 rpms or so to do 60 mph or whatever it is (have to re-do speedo) to get better gas mileage than running in overdrive and he said that was nonsense because it drops the rpm's down to around 2000 when in overdrive and than means better MPG. I told him that is a very simplistic way to look at it because RPM's are not the sole factor. He is convinced that rpms are directly related to gas mileage and how much gas you use. Explain to me so I can explain to him and end this stupid debate which is pissing me off.....the stuborn fool

running at a higher RPM while going at the same rate of speed as before will eat up more gas( this is the point of overdrive..to run at a lower RMP to increase milage at a sacrafice of responsivness). with lower gears (4.56) your rig will be more responsive and feel "torquey" vs. higher gears(3.55) which will make you feel more "sluggish" and require you to stay in lower gears until you can get up to speed. simply changing the gears won't just get you better gas milage, it's all about a ratio, with a certain size tire you try to get a certain ration of tooth so that its roughly the same ratio so the engine can turn the new meats with the same authority as stock, quite a few guys run lower for crawling/offroad situations

running at 2500 RPM isn't that high, but say it was 3000 RPM, if you and a friend had the exact same rig with the exact same tires engine etc and he drove the same distance as you did ( like a long highway trip) and he drove at 2100 RPM he would get a slightly better MPG

Samman said:

running at a higher RPM while going at the same rate of speed as before will eat up more gas( this is the point of overdrive..to run at a lower RMP to increase milage at a sacrafice of responsivness). with lower gears (4.56) your rig will be more responsive and feel "torquey" vs. higher gears(3.55) which will make you feel more "sluggish" and require you to stay in lower gears until you can get up to speed. simply changing the gears won't just get you better gas milage, it's all about a ratio, with a certain size tire you try to get a certain ration of tooth so that its roughly the same ratio so the engine can turn the new meats with the same authority as stock, quite a few guys run lower for crawling/offroad situations

running at 2500 RPM isn't that high, but say it was 3000 RPM, if you and a friend had the exact same rig with the exact same tires engine etc and he drove the same distance as you did ( like a long highway trip) and he drove at 2100 RPM he would get a slightly better MPG

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no, it won't.
Say it with me:
RPMS ARE NOT AN INDICATOR OF GAS MILEAGE
gas mileage is a factor of how effiently the motor is working, not at what RPMs it's running at.
The best indicator of this is a manifold pressure guage. Ever see a BMW with the little gas mileage indicator in the dash, that's a manifold pressure guage.
In order to get the best gas mileage you need to run the motor in it's torque peak, for a 4.0 that's somewhere abouts 2500 RPM, maybe a little higher.
It's in this RPM range that the motor will do it's work most efficiently.
Not only will regearing get you acceleration back, your mileage will go up, because you won't be lugging the snot out of your motor when you accelerate because your first gear is now too tall. Your AW4 will also tank you, because it's slipping the torque converter quite a bit when you are in 1st and 2nd.

Regear it, you'll be happy.
And for all the nay sayers, my 87 Mj got 22 MPG on the way back from missouri, running between 2500 and 3K RPms for 9 hours straight with 3.73s and 31s on the highway, loaded with 1200 pounds of gear in the bed.
If you'd like the numbers, do a search fo volumetric efficiency, you'll find many posts discussing this very topic, and many of them will be by Dino, the resident 4.0 stroker guru.

Your point is right for sure, efficiency is the factor.. but you said it yourself...

"In order to get the best gas mileage you need to run the motor in it's torque peak, for a 4.0 that's somewhere abouts 2500 RPM, maybe a little higher"

therefore by this logic there is a optimum RPM range for the best gas milage correct? and going with this assumption too high RPMs that are not in this range will eat up the gas as the engine is not operation at peak efficiency

RPM and economy isn't perfectly inversly proportional( ie increase RPM=decrease economy) there is most definetly a range of best economy

i should have stated it can work both ways, too high or too low will decrease fuel efficiency and exactually how high or RPM is too high( which i don't know) and i apologize for that

From GoJeep.com:

Gearing for best MPG

Most people think that less revs will equal better MPG. That is not always the case as less revs can also mean more strain on the engine which causes more fuel use. According to a engine design text‚ maximum fuel economy occurs with open throttle and at a piston speed of 1200 – 1500 ft/min. So if you know the stroke of your engine‚ you can calculate the rpm for best fuel economy. Usually comes out somewhere between 2000 – 3000 rpm. Dino provided me with the following numbers. The 4.0L has a stroke of 3.411", so the numbers would be as follows: 1200-1500ft/min piston speed = 2110-2640rpm on a 4.0L.​

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Madness.......Should have known you would have a technical spec write up

OK, so when I'm going down hill, I want to put push in the clutch, and keep the engine reving 2500RPM?:roll:

2001XJ said:

Madness.......Should have known you would have a technical spec write up

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Not mine...I just know where to look.

Ive been having this dilemna myself....
http://www.naxja.org/forum/showthread.php?t=908400
You would think that its a function of AFR and engine RPM, once in the top gear locked that once you hit the peak AFR (you want to highest amount of air/fuel) then you're set. Say at 45mph in 4th gear you may have a lower AFR than at 60mph due to better efficiency of engine, but then again at 90mph you may be near that "peak" zone but the energy to keep the vehicle moving is too great and you start running richer than say at 45mph. Basically in 4th gear locked its linear the amount of miles you travel for a certain engine rpm, BUT the fuel consumption/rev is not linear so you need to find the peak...apparently "2110-2640rpm on a 4.0L. ". What about a 4.0 with mods? And is that formula specific to all sorts of valvetrains because it may be different for someone with DOHC vs an older school OHV. Just a though. But I like to run it at 80mph, 2500rpm or so and I dont think I do too bad.

thanks for the info guys.....if there is any other info.....post it up!

I'm curious, now that I jacked her up and threw on 33's, my mpg is about 9.5 from the 12 it was... since the speedo is off by ~10% would that mean my mpg calculation is also off?? since I'm rolling further then the odo is ticking?

Google online for a speedometer correction calculator that'll ask you for your old tire size to the new one, and your gearing, etc. Your mpg should also drop with the new 33's if you're going bigger due to more rolling resistence and making the engine work harder.

xjbubba said:

OK, so when I'm going down hill, I want to put push in the clutch, and keep the engine reving 2500RPM?:roll:

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If you want to blow your clutch when you re-engage it, then yes! :explosion

(dammit, we really need a pirate-esque flipoff smiley)

I would figure if you let the engine rpm's equal to whatever it should equal at that road speed you could just completely let go of the clutch pedal and it has no effect. I tried this with my friend's accord to see if going 20mph to put it in 2nd gear so I revved to whatever it was, maybe 3k with pedal all the way down then completely let go at 20mph and it didnt buck, jerk or anything. I would ASSUME that it both ends are moving at the same speed there shouldnt be any reason for damage. Someone correct me if im wrong.