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HOW TO: Improve your MPG


This is a discussion on HOW TO: Improve your MPG within the How To forums, part of the Jeep Liberty - KJ (02 to 07) category!
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In this section it explains how to do installations and modifications.


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Old 08-05-2006, 08:05 PM   #1 (permalink)
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Default HOW TO: Improve your MPG


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I liked my reply to a post so much I decided to turn it into a How To.

This post is brought to you by Scotch Ale, made by your friends and mine at Depot Street Brewing. So dark, not even light can escape (not their official motto, but that's how I've been pitching it).

DISCLAIMER: I have a tendency to be brutally honest (especially when I've been drinking). Some people may take it the wrong way, but remember, it's nothing personal, I'm just an ass (especially when I've been drinking).

Most manufacturers have already done everything they can to improve your MPG. These days, it's a great selling point. Translation: you will sell more vehicles if they have better fuel economy. They only limiting factor are those bastards at the EPA and their emission standards.

There are 6 things you can do to improve you fuel economy:

#1 Change your driving habits. Slowly accelerate from stops, don't accelerate uphill, coast downhill as much as possible, draft, and don’t go above 60 mph.

#2 Increase your airflow by adding an aftermarket intake and exhaust (with mandrel bends). (There isn't much cost benefit here, MPG increase is minimal and parts aren't cheap). This will give you a slightly leaner air/fuel ratio (ideal is 17:1). This ensures there is enough oxygen present to achieve as close to 100% combustion as possible. There will be an increase in H20, CO2 emissions along with NOx (NO, NO2 and N2O), and a decrease in non-combusted hydrocarbon emissions. Any change will be minimal and SHOULDN"T (but might) affect emission tests. An increase in air also increases the number of particles you let into the engine, decreasing engine life. Again the effects are minimal.

#3 Add an underdrive pulley. Basically, this robs power from other components on the drive belt and delivers it to the engine. Those components include: your A/C (so it won't cool as well), power steering, and your alternator (so your battery won't recharge as quickly). Not very cost effective and you gains will be minimal. The pros might not outweigh the cons if you like your A/C or you pull a lot of current.

#4 Improve your aerodynamics. Lower the front end and spray your entire vehicle with Teflon. You chose the wrong vehicle for aerodynamics, and it's not a very cost effective solution.

#5 Lose some weight. Don't carry around unnecessary weight. You lose 1% economy for every 100 pounds. Go on a diet, take a dump, whatever it takes to drop a few pounds (pun intended). For more extreme results Lose the A/C compressor, radio, roof rack, spare tire and jack, all the seats (except the driver's seat), and all of your interior molding and upholstery. You could probably eliminate 250 pounds and gain .5 mpg efficiency. But think of all the money you'd make selling parts of eBay!

6 Move to sea level, or below (stay above the water please). You lose 3%-4% economy for every 1000 feet you are above sea level (unless you have a supercharger or turbocharger to cram more air in).

I'd like to thank my friends Copy & Paste, Microsoft Word, SpellChecker and alcohol, the cause of...and solution to, all of life's problems.
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Old 08-05-2006, 11:29 PM   #2 (permalink)
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I say =D> and it's all thanks to
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Old 08-06-2006, 10:02 AM   #3 (permalink)
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Default Re: How to Improve your MPG

Originally Posted by VTNomad
#2 Increase your airflow by adding an aftermarket intake and exhaust (with mandrel bends). (There isn't much cost benefit here, MPG increase is minimal and parts aren't cheap). This will give you a slightly leaner air/fuel ratio (ideal is 17:1). This ensures there is enough oxygen present to achieve as close to 100% combustion as possible. There will be an increase in H20, CO2 emissions along with NOx (NO, NO2 and N2O), and a decrease in non-combusted hydrocarbon emissions. Any change will be minimal and SHOULDN"T (but might) affect emission tests. An increase in air also increases the number of particles you let into the engine, decreasing engine life. Again the effects are minimal.

6 Move to sea level, or below (stay above the water please). You lose 3%-4% economy for every 1000 feet you are above sea level (unless you have a supercharger or turbocharger to cram more air in).
Actually, the #2 is only partially correct and #6 is incorrect.

More airflow simply mean your computer adds more fuel (more power, but not better gas mileage). This idea goes back to the old carburated days when there was no computer that meticulously monitored your fuel/air mixture and adjusted for conditions each millisecond. Back in the carburated days adding more air could lean up the mixture a tad and achieve slightly better economy. Combustion efficiency was no where near the 99% level it is today. An aftermarket air filter or high-flow intake really won't do anything from gas mileage despite with people say. You can always get more efficient airflow by opening up the throttle.

Decreasing backpressure can help gas mileage, beause it decreases the load on the engine. But in reality if you are cruising at 60mph and at 1800rpms the stock muffler is not that restrictive.

Higher altitude does NOT decrease gas mileage. You lose 3-4% power for every 1000 feet but not gas mileage. This is exactly the same argument as the air filter/high flow intake. You computer will adjust for the altitude and use less fuel. You will have less available power, but not less gas mileage. This idea also goes back to the carburated days when there was not really a way to dynamically compensate for changes in climate.

Actually, fuel economy can be slightly improved with altitude. Less air density means less air drag. At 5000 feet the density of air has dropped 10% hence the force of air drag has dropped 10%.


Modern automobiles burn about 99+% of the fuel injected into the cylinders. Combustion efficiency is about as high as it can be achieved. Other areas such as thermal efficiency, rolling resistance, drive train loses and air drag cause almost all the loses.
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Old 08-06-2006, 01:06 PM   #4 (permalink)
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Default Touche

Good points.

Your computer can only add fuel too a point. A larger fuel pump and large diameter lines will be necessary.

I'm not sure why I was thinking altitude hurt your fuel economy. It hurts you horsepower. Alcohol has a way of playing tricks on you like that.
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Old 08-07-2006, 01:56 AM   #5 (permalink)
 
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I don't know if it's the same with the automatic transmission, but I would change line #1 to specifically state that the reason not to go fast is because the engine will rev higher. I'm not sure the exact level, but I heard that the 3.7L engine was most fuel-efficient at around 2200RPM, which for me is 6th gear at 65mph. I've tried going various speeds on the highway and resetting the overhead MPG gauge and found that 65mph is optimal (for 6th gear).

So to say it a different way, I actually get better gas mileage going 65mph than going 60mph or 70mph.
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Old 08-07-2006, 05:02 AM   #6 (permalink)
 
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Originally Posted by Se7enLC
which for me is 6th gear at 65mph.
Pretty much the same for the 3spd auto with OD on. I haven't actually tried driving extended distances at 65mph, but at 75 with cruise I can avg. around 22mpg. May have to take a road trip to see what 65mph does for me...
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Old 08-07-2006, 05:26 AM   #7 (permalink)
 
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Originally Posted by Corwyyn
Originally Posted by Se7enLC
which for me is 6th gear at 65mph.
Pretty much the same for the 3spd auto with OD on. I haven't actually tried driving extended distances at 65mph, but at 75 with cruise I can avg. around 22mpg. May have to take a road trip to see what 65mph does for me...
With my old red 3.7L Liberty...

65MPH would give me 24.0MPG.
70/75MPH would also give me 22.XMPG.

With this one? I don't know... probably the same. Grr.
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Old 08-07-2006, 07:42 PM   #8 (permalink)
 
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Here's an intresting site that talks about a geat deal of the myths and such out there, like the magnets, typhoons, etc...

http://www.fuelsaving.info/
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Old 10-26-2006, 01:59 PM   #9 (permalink)
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#7. Get another vehicle if you are worried about gas mileage
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Old 10-26-2006, 02:31 PM   #10 (permalink)
 
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Originally Posted by KJgirl
#7. Get another vehicle if you are worried about gas mileage
=D>

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