What RPM should my 4.0 be running at?

What's up with the better mileage at higher elevation? I can get 18+ MPG running 35s on mountain highways in SW Colorado, but struggle to get 13 MPG down on the plains.

I've always assumed it has to be the fact that for every mountain you go up, you go back down and using almost no fuel on the downhill more than offsets the extra you had to use to get up.

The only other thing that could possibly explain it is maybe there's something in the ECM fuel tables, or missing that causes it to lean out a bit at elevation. I'm not an EFI guru, so it comes with a grain of salt, but I would think that since MAP stands for manifold absolute pressure, it wouldn't care what elevation you were at. There's no humidity sensor, and air density does increase with decreasing humidity, so it might lean out in very dry air as is common in the Colorado Rockies. But it would be such a very, very small amount that I don't think it would produce a noticeable change in fuel economy...especially since the EFI engineers didn't think it necessary to adjust for in the maps.
 
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What's up with the better mileage at higher elevation? I can get 18+ MPG running 35s on mountain highways in SW Colorado, but struggle to get 13 MPG down on the plains.

I always thought that it was because, for me, I drive slower at higher elevations. My engine most definitely chokes harder at 6000'+. I can tell because I live at near sea level and get maybe 10 MPG on the freeway as a base.
 
I rarely take mine past 2000 RPM's
I usually shift just under 20000 RPM's
Consistently drive around 1500-1750
But I have only had mine for about 5 months now
Guess it wont last very long
 
Anything below 2600 RPM is lugging the engine.


Ok.....got the 4.0 and I have 35" tires...and a 5 speed

Running it up into the 3500 range between gears is not an issue. But getting there in 5th is NOT going to happen. At "highway speed" (since my speedometer lies, and at this speed about half the vehicles are with me and half are passing me) in 5th I would be good to run 2500 rpm. Getting it up above that speed is, to me, pushing the limits of the ability to stop and/or maneuver in any sort of sudden manner.
 
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I've always assumed it has to be the fact that for every mountain you go up, you go back down and using almost no fuel on the downhill more than offsets the extra you had to use to get up.
...

In the mountains I get to coast a lot and often pick up speed that gets me up and over the next hump. That's the best I have come up with. I try to pay attention to the full round trip rather than the big downhill drive home. Still, the mileage is often higher than in eastern half of the state.
 
Higher altitude has less oxygen in the air resulting in less fuel being used to maintain the proper mix. This produces less power, which usually requires more throttle to maintain a given speed.

If your EFI computer is working perfectly there should be no real change in mpg. But it may just be more efficient at higher altitude for whatever reason.

The other factor is thinner air offers less wind resistance. A good chunk of power produced by the engine at highway speeds is used to counter wind resistance. And with a brick like the Wrangler that may have a pretty big effect on mpg.
 
Yep, thinner air will make the PCM lean the fuel mixture. Even though you are using more throttle going up hills, there is less air getting into the motor, so probably a wash in total fuel consumption vs lower elevation. Every hill you climb means a coast on the other side, so that saves fuel. And you are likely driving slower on those winding roads too. Thinner air and slower speeds = much less wind resistance = less power/fuel needed to move the vehicle.

If you could have a flat plain at 15000 feet and drive slower like you do in the mountain roads you'd probably get great gas mileage...and the engine would be a gutless turd as well. The altitude sickness sucks though. We were hiking in Rocky Mountain NP around 12,000 feet when I suddenly got light headed, dizzy, nauseous, slurred speech. We immediately went back to Estes Park (8,000 ft), still felt queasy and light headed. Didn't feel back to normal til we returned to Loveland (5,000 ft) the next day.
 
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The taller your tires, the less room there is for compromise. Running 35s or 37s you need to be geared to produce more power than running 32s or 33s. Overall statements like, 2600rpm or you are lugging it make no sense. On a flat, tailwind, downhill run there is nothing wrong with 1800rpm as long as you are just cruising and not needing to accelerate.

But I like gearing which lets me cruise, 60mph, close to 2000rpm with 33s. And I shift before it starts to lug. If you are running 37s there is a lot more inertia to overcome just to keep them spinning at 60mph and deeper gearing to achieve closer to 3000rpm at 60mph is a definite requirement.

Transmissions for the past 20 years have been delivering lower and lower rpms for highway cruising. The 8speed double overdrive transmissions often achieve top speed in 5th gear, losing speed in 6th, 7th, 8th gear. The top gears are designed to lower rpm, increase fuel economy, extend engine life, deliver a quieter ride..... and they shift without driver input.

If you just want to put it in high gear and forget about shifting, yes gear for 60mph and 3000rpm. But that is the attitude which has seen the demise of the manual transmission. Knowing a computer can manage shifting better than 95% of the drivers..... the clutch has virtually disappeared.
 
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Higher altitude has less oxygen in the air resulting in less fuel being used to maintain the proper mix. This produces less power, which usually requires more throttle to maintain a given speed.

If your EFI computer is working perfectly there should be no real change in mpg. But it may just be more efficient at higher altitude for whatever reason.

The other factor is thinner air offers less wind resistance. A good chunk of power produced by the engine at highway speeds is used to counter wind resistance. And with a brick like the Wrangler that may have a pretty big effect on mpg.

It would be interesting to compare high Rocky Mountain mileages vs Appalachian. My flatland altitude driving starts where the Eastern high mountain driving ends. I still think it's a matter of exploiting the actual landscape.
 
Anything below 2600 RPM is lugging the engine.
'02 Sahara, 75K miles, 4.0L, 32RH 3 speed automatic, 3.07 gearing on stock 30" tires. I've been watching the shift points of the automatic and it only hits 2600 rpm when I am "urgent" with the skinny pedal, often staying below 2200 and below 2000 in traffic. Is there something wrong with my tranny that I should have it serviced? Don't want to build up carbon deposits in the engine.

A full throttle take-off will get the engine up to 3000 rpm. Also, most common speed is 60 mph and 2200 rpm; the fastest I go is 65 and about 2300-2400 rpm. I will be stepping up to 31" tires in the next few weeks and have been advised that the stock gearing will be fine, now I am thinking otherwise.

Thoughts from the guru's?
 
'02 Sahara, 75K miles, 4.0L, 32RH 3 speed automatic, 3.07 gearing on stock 30" tires. I've been watching the shift points of the automatic and it only hits 2600 rpm when I am "urgent" with the skinny pedal, often staying below 2200 and below 2000 in traffic. Is there something wrong with my tranny that I should have it serviced? Don't want to build up carbon deposits in the engine.

A full throttle take-off will get the engine up to 3000 rpm. Also, most common speed is 60 mph and 2200 rpm; the fastest I go is 65 and about 2300-2400 rpm. I will be stepping up to 31" tires in the next few weeks and have been advised that the stock gearing will be fine, now I am thinking otherwise.

Thoughts from the guru's?
I'd regear to a lower ratio. 3.07 is bad enough with 30" tires, it only gets worse with larger tires. Whoever said stock gearing is fine probably didn't realize or think about that you might have the dreaded 3.07 ratio.
 
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I'd regear to a lower ratio. 3.07 is bad enough with 30" tires, it only gets worse with larger tires. Whoever said stock gearing is fine probably didn't realize or think about that you might have the dreaded 3.07 ratio.
I would have to second this as well.
 
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I'd regear to a lower ratio. 3.07 is bad enough with 30" tires, it only gets worse with larger tires. Whoever said stock gearing is fine probably didn't realize or think about that you might have the dreaded 3.07 ratio.
Can you help point me in the right direction as to a new ratio and axle rebuild requirements (if necessary)? I have been reading a lot on this forum but so much depends upon your specific set up and use.

My TJ is a daily driver and sees much more asphalt than dirt (95/5 or maybe even 98/2). When I do get off pavement it is to meander along sandy forest roads with the occasional washout to climb, easy on the throttle and easy on the land. Highway speeds are typically 60 mph, rarely above 65 (too loud for the wife). I have installed +2" ProComp coil springs and Rancho shocks but the only planned increase is the 31" tires. There is a possibility that I might install a 1.25" BL and 1" MM lift then go with 32 or 33" tires once the 31's wear out that, but that is a maybe.

Would new gears alone be adequate? Should I go with the Super 30/35 rebuilds? Can I do those in steps, ring and pinion now, axles in a year or so? I'm still saving to replace the worn tires and do not have the money to do everything right now.
 
I've always assumed it has to be the fact that for every mountain you go up, you go back down and using almost no fuel on the downhill more than offsets the extra you had to use to get up.

The only other thing that could possibly explain it is maybe there's something in the ECM fuel tables, or missing that causes it to lean out a bit at elevation. I'm not an EFI guru, so it comes with a grain of salt, but I would think that since MAP stands for manifold absolute pressure, it wouldn't care what elevation you were at. There's no humidity sensor, and air density does increase with decreasing humidity, so it might lean out in very dry air as is common in the Colorado Rockies. But it would be such a very, very small amount that I don't think it would produce a noticeable change in fuel economy...especially since the EFI engineers didn't think it necessary to adjust for in the maps.


I've thought similar! When I head out to the desert, I'm always nervous with all the hills that I'm going to burn through gas faster. Especially since a desert trip means I'm fully loaded with a RTT. Yet somehow.... when I take the mountain pass route, I get better gas mileage than the "easy" around the mountain route.


On the other comments... I'm going to have to pay really close attention to my shifts and cruising speeds driving home tonight. I always feel like all my power is down low, not as high as everyone else is stating. I do get it up over 4,000 RPM's regularly to keep the deposits low, but I also don't feel like I'm lugging it at RPM's below 3,000 like Jerry stated.
 
I'd regear to a lower ratio. 3.07 is bad enough with 30" tires, it only gets worse with larger tires. Whoever said stock gearing is fine probably didn't realize or think about that you might have the dreaded 3.07 ratio.
'02 Sahara, 75K miles, 4.0L, 32RH 3 speed automatic, 3.07 gearing on stock 30" tires. I've been watching the shift points of the automatic and it only hits 2600 rpm when I am "urgent" with the skinny pedal, often staying below 2200 and below 2000 in traffic. Is there something wrong with my tranny that I should have it serviced? Don't want to build up carbon deposits in the engine.

A full throttle take-off will get the engine up to 3000 rpm. Also, most common speed is 60 mph and 2200 rpm; the fastest I go is 65 and about 2300-2400 rpm. I will be stepping up to 31" tires in the next few weeks and have been advised that the stock gearing will be fine, now I am thinking otherwise.

Thoughts from the guru's?
Your 3.07,s are designed for higher top end speed, anyone would think it was a major design flaw or cost cutting exercise but its not!
Ask yourself what you want. faster acceleration with high RPM,s or more top end speed with lower RPM,s to get there.
How many dead 4.0l with 3.07,s because the rev range was too low? must be all of them by now lol
If you want 33 or 35" tires then you will need a re gear because your TJ wasnt designed to use 33 or 35" tires.
Do a bit of reading up on long gearing and short gearing before you jump into re gearing.
Buy a set of second hand 31,s on rims and try them on your TJ, there should be loads for sale cheap given so many are upgrading to new wheels and 33/35,s.
If you need to save up for tires then you could be stuck with an expensive mistake if you buy the wrong size! If you buy stock you will always wish you got 31,s if you get 31,s or bigger and need a re gear you will wish you stayed stock.
Now if money is no object just go 33,s with a re gear.
 
that's exactly what I'm thinking about when I think Jeep TJ


it's a major design flaw whether Chrysler intended it to be or not
Maybe to you? my 3.07,s with my 6 speed I Change at the revs that sound right and that's just over 2000 RPM, If I gun it I change at 2500RPM no higher because no higher is needed.
Miss matched wheel and tire size to gear ratio is the problem not gears installed at factory!
The lack of 3.07 power on bigger tires is because 3.07 revolutions of a drive shaft doesn't turn an oversized tire one full revolution like it does a recommended max tire size, its that simple. :)
Flame away lol