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Old 07-14-2012, 11:27 AM   #1
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Interesting MPG Experiment today

I decided to put using 5th gear to test on the highway in my 99 4.0 with 3.07 gears and 33" Duratracs. We all know it is hard to maintain highway speed in the 4.0 in 5th gear so I wanted to see what got better gas mileage, using 4th gear in the power-band or using 5th gear with the engine bogging down. Here are my results

I maintained 65mph on the highway (my speedo is off as I never changed it when I got bigger tires) and 55 on the backroads. I drove the same exact 45 miles (I had to drive to my brothers house to pick up my dog).

RESULTS:
4th gear - 12.4MPG Round Trip
5th gear - 14.9 MPG Round Trip

The only thing that was different between the two trips was that I got behind an RV for 10-15 miles on the way back (5th gear) so it may have benefited from a little bit of drafting.

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Old 07-14-2012, 11:37 AM   #2
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Running in the power band at the same speed will never give better mileage, just more power. More Rpms per given speed will always use more gas. There are engine and trans efficiencies that go into play but this is a good rule of thumb. You also proved it. Same reason 3.07 axle gearing got the best mileage for stock tjs. Same reason I can still pull 17 mpg on the highway with those same gears and 33s. Big engine, low Rpms. Less gas.

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Old 07-14-2012, 11:58 AM   #3
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Try it with lots of hills, or a stiff head wind. 4th will get better mileage in that case, assuming you can even maintain speed in 5th.
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Old 07-14-2012, 12:02 PM   #4
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There were a few hills but nothing major. Still had a hard time keeping 65 in 5th. Just had the pedal to the floor most of the time. It was definitely more enjoyable driving in 4th but with almost 3mpg difference on such a short trip I will take the cons of 5th gear.
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Old 07-14-2012, 12:14 PM   #5
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I've always wondered, is it bad for the engine to be bogging down for long periods of time?
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Old 07-14-2012, 12:15 PM   #6
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I've always wondered, is it bad for the engine to be bogging down for long periods of time?
Yes.
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Old 07-14-2012, 12:38 PM   #7
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Quote:
Originally Posted by jeepwayoflife
I've always wondered, is it bad for the engine to be bogging down for long periods of time?
Define bogged down. We idle offroad and crawl all the time. It's no different in my mind, the I6 is built for torque at low rpms, so I don't understand how "lugging" the engine hurts it. Also I'm sure your not lugging the engine doing 65. What rpm range are you in at that speed? I will say that running at extremely low Rpms is bad but 2200 or above is fine for long periods of time IMO.

My jeep locked up in 3rd with 3.07s still hits about 2200-2500 at highway speeds. I get wonderful mileage for what it is.
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Old 07-14-2012, 02:31 PM   #8
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when the engine is straining to maintain vehicle speed, or if it can't maintain vehicle speed, that is "lugging" the engine.

And that's how you can lug an engine at speed.

The dangers of lugging your engine - Bob Is The Oil Guy

Here's a discussion about it, but an analogy that might make a little more sense is riding a bicycle uphill in too high a gear. As you continue the climb your legs (the connecting rods and pistons) will start to strain/burn because of the effort. Your engines pistions will start to scuff the cylinder walls if allowed to go too long. Because of the increased friction of the pistons, the crank bearing start to take increased loads as well.

Not a good scenario, not good for the engine.

As long as the engine is operating in the power band, and has enough "overhead" power (extra power to accelerate), then you and your engine are fine.
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Old 07-14-2012, 02:35 PM   #9
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Ahh the good ole days of somewhat decent interstate mileage...

I use 5th every chance I get with my lift and tires. 6th gear was not a possibility a long time ago on the JK.
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Old 07-14-2012, 03:14 PM   #10
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Probably a "well duh" for most of you, but I experimented keeping speed down in my 08 auto. I commute from Norman to Dallas regularly and typically got 12.4 -15.8 mpg going ~68-72mph.... drove down going 62 and finished the trip with more than half a tank @ 22.8mpg with indifferent windage. I may take it easy for now on when safe to do so. I guess the weak 4 speed revs too high after 65.
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Old 07-14-2012, 03:28 PM   #11
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Lugging increases internal cylinder head pressure.
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Old 07-14-2012, 03:32 PM   #12
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I am the type that will call BS when I see it,with that I also did an experiment.
I have an "06" unlimited 4.0 six with Auto.3.73's

I have been running 180 miles a day round trip and it was starting to hurt.
I usually roll @ 75-80 Mph and was getting right around 13 to 15 Mpg.
I dropped my speed to 58 Mph and set the cruise and couldn't believe what I saw.
So I did it for the next three days.
Speedo matched my GPS
Avg was 22.1 Mpg.
Funny thing I called someone on this just last week,go figure!
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Old 07-14-2012, 03:36 PM   #13
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Quote:
Originally Posted by div4gold
Lugging increases internal cylinder head pressure.
Can you give am exact description of lugging?
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Old 07-14-2012, 04:07 PM   #14
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Can you give am exact description of lugging?
For me, lugging is when you are in too high a gear to accelerate. Take for example if you're in 5th gear at 40 mph. If you want to keep going 40, level ground, that's fine. If you hit a hill and all of a sudden want to hold 40 or go 50, you push the gas and - - -pretty much nothing happens. Used to be in older cars the valves would start rattling.

My definition- -not really technical, just seat of the pants.
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Old 07-14-2012, 04:42 PM   #15
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Quote:
Originally Posted by GoldenSahara00 View Post
Can you give am exact description of lugging?
Quick engine 101.
The crankshaft has holes in it that are timed to provide a shot of oil right when the pressure on the rod bearings are low. Then the hole rotates and traps the pressure. The rotation of the crank creates a pressure wedge that keeps the bearing supported and off the crankshaft journal. There is a bleed off of pressure from the side clearance. Now when you open the throttle at too low of a speed cylinder chamber pressure increases pushing down on the rod bearing harder to accelerate the car. If you are in too low of a gear you will not accelerate and what happens is the oil pressure wedge will bleed out and the rod bearings can come in contact with the journal. If you accelerate quick enough the pressure wedge does not have time to bleed off and all is well inside.
That is the quick answer.
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Old 07-14-2012, 06:41 PM   #16
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^ so that means that if you are turning the engine at too slow of a speed (rpm) the engine will not supply enough oil to lubricate the cylinders?

When would that occur on the 4.0?

What about just driving slow and not trying to accelerate?
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Old 07-14-2012, 09:39 PM   #17
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No that only means that the rod bearings are not going to have pressure long enough at low rpm/ High pressure. The cam, lifters and cylinder walls will still have normal oiling. Normal crawling or low speed driving is different.You have lower cylinder pressure. But when you open up the throttle and the blade snaps to wide open the engine gulps in a big breath of air along with a bunch of fuel and bingo pressure jumps. Being in high at 20 mph and flooring the gas is bad if you don't downshift. That is an extreme situation but you get the idea.
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Old 07-14-2012, 10:38 PM   #18
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Good explanation, and yes I understand what lugging feels like. I find it difficult to lug in my automatic because when I hit the throttle it does that cool thing where it shifts for me or the TC kicks in. In a standard I definitely see the issue. What do you think the proper running range of Rpms for highway would be? And I feel like the "lugging" isn't as much an issue as the acceleration that may take place afterward.

Anyways again and awesome engine 101. Thanks.
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Old 07-14-2012, 11:13 PM   #19
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I think the power band curve is not the same as the efficiency curve.
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Old 07-14-2012, 11:25 PM   #20
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Quote:
Originally Posted by rrich
I think the power band curve is not the same as the efficiency curve.
I do think I stated that, though not directly. I never made any claim to know the exact numbers but my personal experience agrees with the OP's.
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Old 07-14-2012, 11:41 PM   #21
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Can you give am exact description of lugging?
In a airplane you have a tachometer and in a airplane with a variable speed propeller you also have a cylinder head pressure gauge. Plus a lot of other different gauges. Picture if you will on takeoff, you have full throttle and full pitch on the prop. If you reduce the rpms and keep full throttle it is still dumping full throttle gas into the engine but the reduced rpm builds up higher internal pressure and can actually blow a cylinder off the engine. Vehicles don't have cylinder pressure gauges but the same thing happens in the engine with more gas and less rpms. You obviously aren't going to blow a cylinder off the engine but you still create higher than normal internal pressure.
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Old 07-14-2012, 11:43 PM   #22
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Quote:
Originally Posted by div4gold

In a airplane you have a tachometer and in a airplane with a variable speed propeller you also have a cylinder head pressure gauge. Plus a lot of other different gauges. Picture if you will on takeoff, you have full throttle and full pitch on the prop. If you reduce the rpms and keep full throttle it is still dumping full throttle gas into the engine but the reduced rpm builds up higher internal pressure and can actually blow a cylinder off the engine. Vehicles don't have cylinder pressure gauges but the same thing happens in the engine with more gas and less rpms. You obviously aren't going to blow a cylinder off the engine but you still create higher than normal internal pressure.
Another good visual to to with the other description. Is there anything inherently bad about that, excess heat, friction, wear and tear? Good information guys.
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Old 07-15-2012, 12:45 AM   #23
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Right an auto makes it really hard to lug a motor. In a dyno situation you usually will find that a motor will be most efficient at torque peak. Highest Volumetric efficiency. I'm guessing 2600 to 3200 for the 4.0. Problem is that just might not be a speed you want to run or are designed for. When you factor in air resistance, rolling resistance, friction in the drivetrain. Most of the time it is a number that is much lower for best fuel economy which is different from engine efficiency. That said, the lightest throttle, and lowest RPM that will maintain the speed you desire will "usually" give the best MPG. As far as Normal acceleration it is easier on a motor than full throttle acceleration which is easier on the motor than lugging.
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Old 07-15-2012, 06:00 AM   #24
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seems to me why you have a clutch and shifter. i live out in a semi-rural area with fairly good distances between towns and it's usually having to go to 1 town for this, another for that. most roads between towns are 45-55 mph and once i get up to speed-i'm in 5th and around 2K rpm. i know if i need to pass or get up a hill, pressing on the gas won't do much-so duh-down shift and get up to speed/rpm and back to 5th. auto trannys do this for you-a manual you have to do it. have a 98 4 banger, 5 spd that's 100% stock and i average in the 20mpg range. in town-stop/go traffic- i never get into 5th, some times never out of 3rd but on the open road at constant speed-always. see this pop up every now and then--beginning to think there are alot of ppl out there with manual trannys that maybe should have automatics

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Old 07-15-2012, 07:45 AM   #25
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I know when I need to downshift...I was simply testing two theories.

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