ideal RPMs/MPG

[fallacist]

I've read on some of the threads in this section that our 4.0 performs best around 2700 RPMs.

I have a '01 on 31's w/ stock gears. I'm at roughly 2225RPM cruising on highways at 65-70MPH. That seems like a good set of numbers to me. Some guys say they drop it in 3rd when they drive around just to get their rpms up higher.

I find it very hard to believe that I would get increased mpg by increasing my rpms, it goes against everything I've ever read about ideal mpg/rpm correlations. Can someone explain this?

 

[Johnnie Walker]

I'm at approx 2350 @ 70 with TC lockup with 31's and 4.10s. I usually average 19+ hwy.
With more rolling mass, sometimes higher RPM is better to keep it close(r) to the TQ range. When I had 31's a stock 3.55s at 70 I was running around 2100rpm. Are you sure your TC is locking up during cruise?

 

[fallacist]

can you explain what you mean by locking up during cruise?

the thread i was referring to was another mpg thread: http://www.naxja.org/forum/showthread.php?t=1081070

I'm giving it maybe half throttle to maintain my rpms and speed.

 

[PetrolHead]

I also have a very hard time believing that higher RPM's equate to better MPG on the highway when cruising. If it's just flat land cruising, how could higher RPM's be better for gas mileage. I also have 31's and 3.55's and mine's right around 2100 RPM at 70.

Locking up during cruise is when the torque converter locks up electronically...essentially making the drivetrain direct drive. It usually happens above 45 and when you're light on the throttle...it feels like the transmission shifts again. It was implemented to reduce RPMs and increase MPG, that's why I'm surprised with all this higher RPM = Higher MPG talk.

 

[fallacist]

ahh, ok I know exactly what that means. I'm pretty sure its locking up, that's what it feels like.

 

[DanMan2k06]

If you're unsure, cruise at a speed where you KNOW it should be locked up (50-60). Tap the brake lightly while still on the throttle. You should see the RPM's rise about 300 or so, and then drop back down. That's the converter unlocking, and then locking again.

 

[Darky]

I find it very hard to believe that I would get increased mpg by increasing my rpms, it goes against everything I've ever read about ideal mpg/rpm correlations. Can someone explain this?

It's easy to test, try it. Run a tank or two where you keep it in 3rd and compare to your mileage with normal driving.

Mine sometimes runs cooler when I keep it in 3rd. Not always, but sometimes.

 

[Jeep450r]

AX-15 4.88's and 33x12.50's turning 2500RPMs at 65mph = just under 19 mpg for me.

 

[Super mud]

31s with 3.55s vs 4.10s proved it to me. It's only a few hundred rpms but make running hills much easier in OD and not that I really use less throttle but the load (vacuum loss?) is less. So Mr.Map says less fuel.

Now at 80 (2500rpms) I have to start getting into the throttle more to support the rev, which in my opinion is too high. Specially the Renix, which starts to scream at those rpms. Other than that it sings down the highway, last trip getting ~23mpg doing 70-75. Actually clicked the pump on 4 more times to make sure it filled it as full as the beginning pump.

 

[O-Gauge Steamer]

Everyone's opinion will be different. From '96 to '01, the Heep's max torque was reached at 3,000rpm. In theory, you will get the best on highway performance whilst sitting on the top of the torque curve as that is where the engine is the most efficient.

But, 3krpm? I run around 2,300rpm with 4:56s and P285/75R16s at 70mph. The highway mileage runs between 19+ to 21- depending. When I go to Denver, it gets the 21ish but when I come back home, I get the 19 or less. It is uphill from Denver to my place. Pretty much the entire drive is uphill which account for the differential. In town, I am looking at 15-. Depending. Depending on if the guy driving the Ricer next to me has me irritated enough to blow him off the line. I run a Supercharger...

But, let's get real here. If you want a vehicle that gets good gas mileage, get a Subaru. My Kid's '00 Subi, that has over 250k miles on it, still gets 30+ on the highway, 25- in town. He lives in Metro Denver as he is going to school there. So that is at one mile of altitude. Who knows how good it would be at sea level. Better, for sure.

With all of the aerodynamics of a Brick Wall, the XJ has never and will never get "good" mileage. It is heavy and under powered in the stock form. But then that is not why we drive them, is it...

 

[muddypunk]

95,HO,ax-15 285/75/16 and 3.73 gears and I get 17 around the island I live on and 21mpg @ 60 mph and when I get out on the freeway I can get up to 23 mpg is my best reading with 3 tanks of fuel on a long commute.Also I have a disco and a fairly unchopped Jeep 4.5 lift but body is not hacked out so cut the air like a box

 

[Super mud]

With all of the aerodynamics of a Brick Wall, the XJ has never and will never get "good" mileage. It is heavy and under powered in the stock form. But then that is not why we drive them, is it...

I disagree that xjs are heavy and underpowered. 3400lb is light for 4 doors and 4wd. I think 240lb/ft is damn good even to todays standards. Think how the Jk owners feel about being underpowered.

 

[streetxj]

I disagree that xjs are heavy and underpowered. 3400lb is light for 4 doors and 4wd. I think 240lb/ft is damn good even to todays standards. Think how the Jk owners feel about being underpowered.

Agree. 3400 pounds is pretty damn light. A fox mustang vert weighs around 3300-3400. According to my title, my 96' 2 door 4x2 cherokee weighs 3200 pounds. Even though I think it's lighter than that, maybe not. That right there is how much a fox mustang 5.0 hatchback weighs more or less.

 

[Johnnie Walker]

Heavy and underpowered is the first gen Exploder. 5000lbs and that Ford 4.0L = fail. Not saying the engine is, but not enough for that heavy POS.