What RPM should my 4.0 be running at?

freedom_in_4low

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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'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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ejay

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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.
 

BlueFl.Wrangler

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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
 
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JTrigg

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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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jjvw

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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.
 

WallyWest

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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.
 

GASnBRASS

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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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billiebob

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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.
 

jjvw

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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.