HOW TO: Improve your MPG

Discussion in 'How To' started by VTNomad, Aug 5, 2006.

  1. VTNomad

    VTNomad Full Access Member

    Messages:
    418
    Likes Received:
    0
    Joined:
    Dec 26, 2004
    Location:
    Johnson City, TN
    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.
     
  2. firefighter5212

    firefighter5212 New Member

    Messages:
    7
    Likes Received:
    0
    Joined:
    Jul 22, 2006
    Location:
    Carnesville, Ga
    I say =D> and it's all thanks to (beer)
     
  3. Eddo

    Eddo Full Access Member

    Messages:
    501
    Likes Received:
    0
    Joined:
    Jan 29, 2005
    Location:
    Dayton Ohio
    Re: How to Improve your MPG

    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.
     
  4. VTNomad

    VTNomad Full Access Member

    Messages:
    418
    Likes Received:
    0
    Joined:
    Dec 26, 2004
    Location:
    Johnson City, TN
    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.
     
  5. Se7enLC

    Se7enLC Full Access Member

    Messages:
    373
    Likes Received:
    0
    Joined:
    Feb 18, 2005
    Location:
    Watertown, MA
    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.
     
  6. Corwyyn

    Corwyyn Full Access Member

    Messages:
    2,540
    Likes Received:
    3
    Joined:
    Feb 5, 2005
    Location:
    Southeast of disorder...
    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...
     
  7. Marlon_JB2

    Marlon_JB2 Kombat Edition Jeep

    Messages:
    15,021
    Likes Received:
    45
    Joined:
    Nov 20, 2004
    Location:
    Clinton Township, MI
    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.
     
  8. Trodo

    Trodo Full Access Member

    Messages:
    621
    Likes Received:
    0
    Joined:
    Jan 14, 2005
    Location:
    Orange County, CA
    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/
     
  9. KJgirl

    KJgirl Full Access Member

    Messages:
    82
    Likes Received:
    0
    Joined:
    Oct 25, 2006
    Location:
    Chester County PA
    #7. Get another vehicle if you are worried about gas mileage O:)
     
  10. MoladoGuy

    MoladoGuy Full Access Member

    Messages:
    3,887
    Likes Received:
    7
    Joined:
    Oct 2, 2004
    Location:
    DeTrOiT (Actually Warren)
    =D>

    X2
     
  11. Back-n-Black

    Back-n-Black Full Access Member

    Messages:
    98
    Likes Received:
    0
    Joined:
    Feb 4, 2009
    Location:
    Tacoma Wa
    Couple things to add.

    1. Tires. While big wide tires might be all the rage for some, knowing what it does for gas mileage is not so good. The wider the tire the more contact and friction it creates and takes to turn it. Also the weight of big tires makes them harder to turn. Wheels and how big a wheel is and how much it weights is the same. Aluminum 15" rims with 225/75/15's on them are much better on MPG than 18" rims with 265/65/18's.

    2. Librication. Just like your happy alone time :D, things just go better with the proper lube. Synetics in all the componites of your drive train can gain you a couple MPG because of the reduced friction and heat.
     
  12. tjkj2002

    tjkj2002 Full Access Member

    Messages:
    10,614
    Likes Received:
    8
    Joined:
    Dec 17, 2006
    Location:
    Somewhere between being sane and insane!
    #1 is totally false.Driving like your 100 years old builds up carbon deposits which will kill your mpg's worse then "blowing the cobwebs out" often.
     
  13. flair1111

    flair1111 Full Access Member

    Messages:
    847
    Likes Received:
    0
    Joined:
    Jul 10, 2007
    Location:
    usa
    please do. ive been wondering myself. i dont have the evic or anything to test myself. fine a nice flat stretch a few miles long. go 75 one way, turn around and try 65 coming back.
     
  14. KJ zGal

    KJ zGal Full Access Member

    Messages:
    472
    Likes Received:
    0
    Joined:
    Nov 21, 2008
    Location:
    Warnerville, NY
    I've seen my gas mileage drop as much as 3 MPG when I drive the Jeep hard--When I drive easy, I've gotten 22.2 MPG out of that 2-ton hunk of metal, and I'm completely serious. I stay within 5 miles of the speed limit, keep my RPMs as close to 1500 as I can, and I coast a lot. I don't draft people--that was proven ineffective on Mythbusters. Most cars reach their greatest MPG between 45-55 mph. Ensuring you're not excessively idling or carrying around any excess weight is also going to improve your mileage.
     
  15. MoladoGuy

    MoladoGuy Full Access Member

    Messages:
    3,887
    Likes Received:
    7
    Joined:
    Oct 2, 2004
    Location:
    DeTrOiT (Actually Warren)
    :eek:

    You have lots of control!! HAHA... It's hard to keep the vehicle at/under 2000 RPM when moving off a light, stop sign, passing, etc....
     
  16. maverick7321

    maverick7321 Full Access Member

    Messages:
    220
    Likes Received:
    0
    Joined:
    Mar 13, 2008
    Location:
    Georgia
    My mileage has not noticeably changed since I replaced the stock 225/75/16 Crapyears that came on my liberty with a set of 245/70/16 Yokohama's.

    I think I can attribute that to the fact that the Crapyear's slid and spun alot and would not grip the road at all and as a result the engine had to work harder to keep me moving at a certain speed and when I finally put on a good set of tires I am not on the gas pedal as much or as hard now.

    So even though I went slightly bigger it has not affected my mileage.
     
  17. Dave

    Dave Administrator Administrator Moderator

    Messages:
    15,590
    Likes Received:
    6
    Joined:
    Feb 22, 2006
    Location:
    on here
    Awesome gas mileage for a 3.7L KJ. Good driving and mastering the advantages of the 5 speed.....

    Dave
     
  18. tjkj2002

    tjkj2002 Full Access Member

    Messages:
    10,614
    Likes Received:
    8
    Joined:
    Dec 17, 2006
    Location:
    Somewhere between being sane and insane!
    I get the exact opposite,if I drive my KJ a little harder it gets better mpg's.I get better mpg's at 75mph then at 65mph,no joke either.The last little trip I took,250miles round trip,I got 21.3mpg's going there while doing 75-80mph,on the way back I slowed to 65-70mph and got 18.6mpg's.Seems backwards but since I can not gear correctly yet I need to maintain higher speeds to keep the trans from downshifting of hills which equals better mpg's.
     
  19. jnaut

    jnaut Full Access Member

    Messages:
    2,237
    Likes Received:
    1
    Joined:
    Apr 17, 2008
    Location:
    Seattle
    All cars have an optimal cruising speed. All vehicles have an optimal cruising speed. There's a point where you're pushing more air (or water for you boaters) out of the way, and it's no longer flowing around you. I don't know what the optimal cruising speed for the liberty is. I've suspected it's around 70mph. the problem is, it's very difficult to do a scientific test without proper control. On one trip, you may exceed 75mph and get great mileage. Come to find out you had a tailwind on that trip. Another trip you slow it down to 65mph, come to find out you had a headwind. Or more complicated, you had a tailwind for 60% of your trip, a sidewind for 25% of it, and a partial headwind for the last 15%. Ambient temperature will affect you, altitude, percent grade up or down.

    Also, for a lot of you guys that have changed to bigger tires, if your tire size isn't programmed correctly in the PCM, your speedometer and odometer isn't reading correctly anyway, so you'd need to adjust for that.
     
  20. tjkj2002

    tjkj2002 Full Access Member

    Messages:
    10,614
    Likes Received:
    8
    Joined:
    Dec 17, 2006
    Location:
    Somewhere between being sane and insane!
    I proved that 75-80mph get's me better mpg's then 65mph on many occasions.Yes vehicles have a optimum cruising speed,that is when stock and it will change when you modify that vehicle.Oh and my speedo has been calibrated and I'm a max of 2mph off from actual speed verified by GPS so I'm no worse then what most are stock,which can be as much as 5% +/- actual speed right from the factory.