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Cruising RPM for gas mileage (4-cylinder)

TJim

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Hey guys!

I didn't want to hijack another thread talking about the 6 cylinder.

Do you know what is the peak RPM for the 4 cylinder engine? (Preferably for the 2.4L)

I have a 2004 2.4L 4 cylinder TJ. NV1500 transfer case. 31" tires. I am trying to decide between 4.56 and 4.88 gears.

Do you know, which would be the best RPM for cruising at around 70 - 75 mph? I know for the 6 cylinder is about 3200 rpms. What about the 4 cylinder?
 

machoheadgames

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I'd go straight to 4.88 for 31's on the 4 cyl, 5.13 if any larger than that.

The 2.4L and 2.5L follow basically the same gearing guidelines. 2.4 is a bit more powerful overall but it is a DOHC design and from my findings (and typical DOHC characteristics), it makes power higher up in the rpm range. At the end of the day it's still a 4 cyl that needs help with tires any bit larger than stock, so I would go 4.88 for sure.

I don't know what the 2.4's redline is, but the torque curve looks to be about 165 ft lbs @ 4000.

Here's some info if interested. The follow similar torque curves actually but the 2.4L makes slightly more power overall, and the 2.4 reaches its peak at 4000 vs the 2.5 at 3500. Still, the gearing advantage makes you often want to drive them at higher rpm rather than staying at the peak torque rpm simply because the lower gear choices are "easier" for the engine to push the vehicle with, despite what rpm they make the most power at.

2.4L Torque Curve

2.5L Torque Curve
 
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TJim

TJim

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I'd go straight to 4.88 for 31's on the 4 cyl, 5.13 if any larger than that.

The 2.4L and 2.5L follow basically the same gearing guidelines. 2.4 is a bit more powerful overall but it is a DOHC design and from my findings (and typical DOHC characteristics), it makes power higher up in the rpm range. At the end of the day it's still a 4 cyl that needs help with tires any bit larger than stock, so I would go 4.88 for sure.

I don't know what the 2.4's redline is, but the torque curve looks to be about 165 ft lbs @ 4000.

Here's some info if interested. The follow similar torque curves actually but the 2.4L makes slightly more power overall, and the 2.4 reaches its peak at 4000 vs the 2.5 at 3500. Still, the gearing advantage makes you often want to drive them at higher rpm rather than staying at the peak torque rpm simply because the lower gear choices are "easier" for the engine to push the vehicle with, despite what rpm they make the most power at.

2.4L Torque Curve

2.5L Torque Curve
Thanks for the info! Quite useful!

What do you think should be my goal RPM for cruising on highway? For gas mileage, comfort, engine life, etc? Cruising at 4000 rpm seems a bit excessive
 

machoheadgames

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Not suggesting cruising at 4000 at all. That's just where peak torque is, so ideally you use a lower gearset to help you reach that rpm easily when you need power. 70 with that setup would be about 3200 rpm.

There was a guy who used to hang out here years ago (rob5589) with the 2.4L and 6 speed and he really enjoyed his 4.88/31's up until he sold it. Nearly the same overdrive ratio as the NV1500 being 0.84 and 0.83.
 
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TJim

TJim

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Not suggesting cruising at 4000 at all. That's just where peak torque is, so ideally you use a lower gearset to help you reach that rpm easily when you need power. 70 with that setup would be about 3200 rpm.

There was a guy who used to hang out here years ago (rob5589) with the 2.4L and 6 speed and he really enjoyed his 4.88/31's up until he sold it. Nearly the same overdrive ratio as the NV1500 being 0.84 and 0.83.
Thanks for the input!

I am going to check my speed/rpms a bit more excessively the next days.

I think it's going to be either 31s/4.88 or 33s/5.13
 

machoheadgames

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Thanks for the input!

I am going to check my speed/rpms a bit more excessively the next days.

I think it's going to be either 31s/4.88 or 33s/5.13
Cool. If there's any chance you're going to kick it up to 33's at all, I would just do the 5.13s. If you do 4.88 and then decide to change to 33's for other reasons, 4.88 will be slightly underwhelming. It will work, but it will be more sluggish than stock was. Better off doing it right to begin with, but definitely need to try and decide on a tire size to make that decision.
 
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TJim

TJim

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Cool. If there's any chance you're going to kick it up to 33's at all, I would just do the 5.13s. If you do 4.88 and then decide to change to 33's for other reasons, 4.88 will be slightly underwhelming. It will work, but it will be more sluggish than stock was. Better off doing it right to begin with, but definitely need to try and decide on a tire size to make that decision.
Would cruising at 3000-3200 rpm be peak for gas mileage with 2.4L engine?
 

machoheadgames

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Would cruising at 3000-3200 rpm be peak for gas mileage with 2.4L engine?
Not really sure on that. I know 4 cylinders like/want/need rpm to maintain speed easily. I have not heard of really any 2.4/2.5 getting bad gas mileage from gearing low. I'd say probably 18 but would be better to hear from someone who regeared a 2.4 and ran it and knows the numbers themselves.
 
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TJim

TJim

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Thanks!

SO, my main question if anyone has the experience is:

Which is the ideal RPM for cruising on highway, as far as gas mileage goes?
 

Fughuert

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On my 2.5l I could never really get more than 13-14mpg on the highway. No matter if I drove conservatively at 65ish which was like 2400 rpm in 5th or just balls out full throttle always.

I had 31s 5 speed 4.10 gears. My main driving style was as fast as the engine will take me. That engine is happy to sit at 3k all day and night and is where I would aim to cruise at with your preferred highway speed.

With that setup I could pretty much always maintain at least 70mph in 4th gear cruise at 78 in 5th(3k rpm). If the winds or downhill allowed I’d take it past 90. Sometimes I’d climb a hill in fourth at 3500rpm for quite a while.

I had 210k miles when totaled and burned no oil.
 
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TJim

TJim

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On my 2.5l I could never really get more than 13-14mpg on the highway. No matter if I drove conservatively at 65ish which was like 2400 rpm in 5th or just balls out full throttle always.

I had 31s 5 speed 4.10 gears. My main driving style was as fast as the engine will take me. That engine is happy to sit at 3k all day and night and is where I would aim to cruise at with your preferred highway speed.

With that setup I could pretty much always maintain at least 70mph in 4th gear cruise at 78 in 5th(3k rpm). If the winds or downhill allowed I’d take it past 90. Sometimes I’d climb a hill in fourth at 3500rpm for quite a while.

I had 210k miles when totaled and burned no oil.
ouch! 13 mpg is low
 

Kon

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Here in Greece we have more 4 cyl TJs than 4.0s.

There is a rumor, which I don’t know if it’s true, saying that gas consumption between 2.5/2.4 and 4.0 is almost identical (everything else equal).

This is based on the fact that the 4 angry squirrels need to press more tha skinny pedal and this higher rpms and more consumption compared to a 4.0 which can achieve the same result with a lighter right foot.

On the other hand though, more displacement usually means higher consumption right?
 
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Gilaguy23

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My 91/2.5 gets 18 in 4th on a 120 mile run of highway. Same trip,similar conditions in 5th I get 15. Id like to swap in a AW4,5th is a no go unless its perfectly flat and still..... I have stock 4.11 gears and 31's.
 

Fughuert

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Here in Greece we have more 4 cyl TJs than 4.0s.

There is a rumor, which I don’t know if it’s true, saying that gas consumption between 2.5/2.4 and 4.0 is almost identical (everything else equal).

This is based on the fact that the 4 angry squirrels need to press more tha skinny pedal and this higher rpms and more consumption compared to a 4.0 which can achieve the same result with a lighter right foot.

On the other hand though, more displacement usually means higher consumption right?
My lj gets similar mileage to my 2.5 on the highway the one time I tested it with 4.10 gears 31s 6 speed.

But the lj cruises at 80-92(fucking limiter)mph with ease getting the same mpg as the 4 cyl struggling to do 70-80.

I haven’t tried to see what kind of mileage I can get doing 65ish.
 
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Petah

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Not sure if it helps this discussion, but I found a very cool site that you can use to calculate gear ratios, RPM at various speeds with various tire sizes. It accepts input for what engine, transmission, transaxle, tire size, differential gear ratio and so on. It also has a cool side-by-side comparison where you can copy all the inputs from side A and paste them to side B, then manipulate data on one side or the other and compare the results. Check out http://www.grimmjeeper.com/gears.html.
 
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TJim

TJim

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Not sure if it helps this discussion, but I found a very cool site that you can use to calculate gear ratios, RPM at various speeds with various tire sizes. It accepts input for what engine, transmission, transaxle, tire size, differential gear ratio and so on. It also has a cool side-by-side comparison where you can copy all the inputs from side A and paste them to side B, then manipulate data on one side or the other and compare the results. Check out http://www.grimmjeeper.com/gears.html.
Thanks! I know this site.

My question though is which RPM I should target for highway cruising. For example, what RPM would it be best for cruising on the highway at 70-75 mph?

Grimmjeeper is perfect, and it is going to help, once I know which my target is!