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Increasing fuel efficiency?

  1. Sep 1, 2005 #1


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    I've been reading a lot lately about how people have vehicles that have a really good fuel economy. Some even seem considerably better than what the normal car can get. It makes me wonder if there are modifications being done to achieve this.

    I drive a 2000 GMC Sierra Z-71 5.3L truck. Please do not send any derrogatory messages saying how its a gas hog, etc. This is what my parents got me for graduation 5 years ago.

    Im curious if there are any driving tips or even relatively inexpensive ($300<) modifications I can make to the truck to improve the fuel economy?

    I was always told that if you are driving in town with a bunch of stopping and starting to drive in 3rd gear instead of overdrive to save gas. Is this true?

    Also I have a hypertech power programmer III chip in it right now. Will this increase/decrease the fuel efficiency under the same driving conditions?

    Just trying to be as conservative as possible to make it through the rough times.

    Any help is appreciated.
  2. jcsd
  3. Sep 1, 2005 #2


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    Leave the truck in overdrive, keeping the RPMs down as much as possible is really important. My brother and I both have 5.7L LT1s and with one a 6spd and the other the same 4spd auto we've learned some interesting things. First the auto can get better mileage around town, 22 or so with lots of interstate, the 6spd only 20. On the highway, its the opposite, 24.5 for the auto and 26-28 for the 6spd (depending on speed, better mileage in WI or KY and so on with lower speeds). If you compare the 1.91 and 1.71 final drive ratios to the mileage difference, its similar to the mileage difference again reinforcing the idea that low rpms=better mileage. With the 6spd the idea of downshifting is out, it hurts mileage too. Kinda like accelerating hurts mileage, we can cut those numbers in half without trying too hard. :smile:

    The hypertech might hurt mileage but its likely it doesn't change it much at all. The maps the computer uses to determine how long to open the injectors uses a lot of factors including the amount of throttle used - I'd assume the programming is more for the higher amount of throttle.

    Driving like granny helps a lot.
  4. Sep 1, 2005 #3
    Not true, chips usually 'tune'---I say that with some disdain BTW---a very small range to the detriment of the rest of your engine operating band.

    TPS is not a feedback sensor usually. When you're driving the position of the TPS does not in and of itself dictate the fuel needs and is only used to determine injector duty cycle when the ECU goes open loop. Even then newer computers still rely on MAF until the engine is at about 80% or so of WAT.

    Driving around your injector duty cycle is determined mostly by the HEGO. The MAF and TPS sensors are used to as predictive sensors and also verify the integrity of the HEGO. When you are cruising along your engine should be running with a mixture of 14.7:1. The HEGO actually oscillates between about .8 and .2V because the slope of the HEGO curve at 14.7 is almost verticle. The base engine needs are determined by the TPS(butterfly moves before there is a change in air flow or manifold pressure) then the MAF sensor(actual airflow into engine) and then finally the HEGO takes almost full control of engine operation(bouncing back and forth around the 14.7:1 afr point).

    Why is that important? HEGO's go bad. They should be replaced every 50K miles becuse the ECU can compensate for small changes but software compensation is not as accurate as an actual physical sensor.

    Clean your MAF sensor.

    Keep your tires at the proper pressure.

    If you have an automatic then change your trans fluid somewhat regularly. Degraded fluid leads to increased slip.

    Change your PCV at regular intervals. I replace mine once a year. Your rings actually seal better when there is a DP across them. Old PCV valves allow the crank case to become pressurized which tends to increase ring blowby so your 'dynamic' compression goes down a little.

    Add a toneau(sp?) cover or an air gate.

    Use periodic fuel injector cleaner.

    Basically, Change your O2 sensor, keep your MAF clean and keep your engine and transmission tuned up and with good fluids.
  5. Sep 1, 2005 #4
    I do not know where you are driving, but if you are in a line of traffic, turn the truck off. Sitting idle will waste more gas than turning the truck off and firing it back up. If you have A/C, it is actually better to drive with the windows up on those long drives. The "wind tunnel" that the truck was put through for aerodynamics was made to maximize fuel effeciency with the windows up. Hopefully this helps you out.

    If it is any consolation, I'm driving a Ford Explorer.
  6. Sep 1, 2005 #5


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    As faust9 says, maintaining your vehicle is the best thing to do to improve efficiency (or well, keep the efficiency from dropping that is). On a side note, I saw an article about how cars are more fuel efficient when they have their a/c while driving at higher speeds and their windows up instead of having their windows open at higher speeds (the drag created by open windows creates more inefficency then the a/c being off). Plus, more aerodynamic cars are more efficient at higher speeds then at lower speeds and visa versa for larger trucks and SUV's and vans.
  7. Sep 1, 2005 #6
    I suspect the AC thing only applies at fairly high speeds. If you're driving around in a city at 30 mph and stopping at a red light every couple of blocks, the aerodynamic drag from having your windows down will be pretty small.

    They did a test of this on mythbusters and the result was that the car with the windows up and AC on ran out of gas first. Their speeds were fairly low though (45 IIRC), so at 75 mph on the interstate it may well have worked out differently.
  8. Sep 1, 2005 #7


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    Yah the article was sayen at freeway speeds (65mph) its more efficient.
  9. Sep 2, 2005 #8


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    faust - no disagreement about the TPS merely being one of many sensors used to determine the base pulse width map and the O2 dominating the overall correction factor applied to the map when in closed loop. But I was viewing it from a more simple perspective of when you open the throttle more the engine is allowed to produce more power and this requires more mixture (gas). And with a 5,000lb truck its easy to slip the converter more and waste more fuel as heat...

    Off topic but isn't Toyota bringing over some super lean 17:1 AFR engine that still has reasonable NOx emissions too? I wonder if they switched to a wide-band O2 to handle lean for cruising and stoichiometric for accelerating or just use a new one calibrated for the higher range?

    50k replacement? Yikes that's expensive for just the 2 primary ones (forget the pair post-cats for OBDII). And come to think of it, I'd thought another O2 had gone bad and was the cause of my mileage dropping slightly, but it may be because my brother had used a Hypertech on my car to try out lowering the cooling fan activation point and I don't know if he removed the program when he was done.

    Good points faust, and excellent maintenance advice.

    Just be very careful cleaning the MAF, the little wire is heated and easily damaged when warm and the unit is not cheap to replace unless you know exactly the model to buy off ebay. GM Top engine cleaner is like $12 in an aerosol can at a dealer, and will not harm the teflon seals on the throttle butterfly. You may want to clean the butterfly too, carbon builds up on it and create turbulence - it makes a surprising difference. Anyways, don't use any carb cleaner, some will hurt the teflon and this can cause a small vacum leak.

    Oh, and check your air filter too.
  10. Sep 2, 2005 #9


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    That is correct. Strategies call that the LAMBSE value (ratio of FA mixture to stoichiometric ratio)which ranged from values of .8 to 1.2 but the obvious goal is to oscillate around 1.0. I can't remember what the oscillation frequency is off the top of my head any more. It was pretty darned fast though. That was all in the HEGOs. Once the HEGOs stop operating the "learned" table is in effect which definitely does not do as good of a job as the sensor feedback even though the table is adjusted for your driving style during operations from the base calibrated table.
  11. Sep 2, 2005 #10
    When you put the pedal to the metal the computer gets the message to completely forget about lean economical fuel mixture and go really rich for the most power. (Somebody may have already said that, but this is the English translation.) So, keep your foot out of it except when ubiquitous semi drivers in semi trucks are about to ram you from behind or some other important emergency.
  12. Sep 2, 2005 #11
    About to leave but thought of one(?) more thing: a cold motor, for several reasons, uses more fuel. Thermostats can wear out to run cold as well as run hot, and a new one can be on the cold side of tolerances: watch the motor temp and you do not want it to always run in the cold area of safe temperatures. In the same thought, be aware that the computer makes a very rich mixture for starting, and for running until temp gets high enough, so get in gear and rolling without sitting and warming up after a start. However, a motor needs to be warm before using a LOT of power, to avoid damage such as cracking a cold ring. If your truck motor doesn't leak or burn much oil use the lightest weight in the user manual and seriously consider paying the price for synthetic oil (I've read so many reports of encreased economy that they must be true. My old Ford ranger leaks so much motor oil that I might as well throw money out of the window as to leak expensive oil.) And I,ve personally experienced as much as 25% and more Interstate mileage improvement driving 55 rather than 75. I am very, very skeptical about any relatively inexpensive add-on for either economy OR power. Use the money for tune ups and maintenance.
    Last edited: Sep 2, 2005
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