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  • Bad Gas Mileage

    Over the past couple tanks of CNG my fuel economy has dropped in half.

    Yesterday I replaced my spark plugs, I hope this will help.

    Any other ideas?

  • #2
    Re: Bad Gas Mileage

    Vehicle make, model, year, miles?

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    • #3
      Re: Bad Gas Mileage

      2000 Cavalier 2.2L Dual Fuel.

      Someone told me that their car (similar to mine) is terrible on fuel in town or if they are going over 70 mph.

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      • #4
        Re: Bad Gas Mileage

        check the o2 sensor

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        • #5
          Re: Bad Gas Mileage

          I'm sure you wouldn't have the AC on this time of year, but having it on cuts my mileage on CNG roughly in half.

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          • #6
            Re: Bad Gas Mileage

            I own a 2001 Cavalier and have found that city driving kills my mpgge. I have also found that driving above 70 mph kills my mpgge. The optimal speed for me seems to be about 45 mph (of course, who in their right mind would drive that slow on the freeway?!). One thing you have to remember is that Cavalier had 3 speed transmissions from 1998 to 2000. From 2001 to 2004 they have 4 speed transmissions. I have a friend that owns a 1999 Cavalier CNG and I get way better gas mileage than he does with my 4 speed transmission.
            Jared.
            Mountain Green, Utah
            2003 CNG Cavalier
            2003 CNG Silverado 2500HD

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            • #7
              Re: Bad Gas Mileage

              I my 2001 cavalier 57k miles with the 4 speed transmission, my first two full tanks of CNG (about 5.2 gge) I got 108 miles in city driving, then 113 miles on the highway. That's about 20mpg.

              I've seen people selling the cavaliers stating that they will get 170 miles per tank and 36+ mpg.

              Is this 20mpg about average on a cavalier, or is there something wrong with the system? Maybe the O2 sensor, but I don't have any historical data on what the mpg was in the past to know if there is something wrong or not. So I'm wondering if this is normal mpg on CNG.

              I expect the 2004 with the ecotech engine and the fuel injected CNG would do a little better, but how much?

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              • #8
                Re: Bad Gas Mileage

                I can not account for the Cavies but city driving is always of question too much stop and go maybe.Yes three speed auto's will get less than the four speeds, my full size Chevy truck gets 18.5 mpg on highway on either gas.City driving in Utah its about 14.5 but then again I'm not driving in NYC.What kind of mileage do you get on gasoline?

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                • #9
                  Re: Bad Gas Mileage

                  That's a good point to check the mpg on gasoline to see if it is different. I would expect that it is a bit lower on cng, I just don't know how much. But running gasoline through my cng car just doesn't seem right, I like the $4 fillup. I'll have to run through a tank of gasoline and see what I get

                  Before I got this car, I looked at the electrics and the browns gas solutions. It appeared that the electrics were only slightly cheaper to run than cng, but only if you didn't add in the cost of batteries which is up to $1/mile. I've seen people spend $6k for a set of batteries (24 at $250 each) only to find them failing at 6,000 miles. Then they sell the vehicle to the next guy. I don't think that is necessary, it's because they don't know how to take care of the batteries (talking lead acid here), lack of balancing circuitry, not keeping them charged, overcharging and gassing, deep cycling when unnecessary, temperature problems, and the wrong type of battery all togtether. But easy for me to say, I've never done it.

                  The browns gas (water to hydrogen generator) guys claim that you have to adjust the O2 to get the best mileage or the on board computer just mixes the fuel richer based on the O2 reading. They have a small 1/2" high standoff that goes under the O2 sensor to pull it out of the way of the air flow on the exhaust manifold so the reading is more accurate, because of the high oxygen level in the exhaust from the browns gas. Also I've seen a neat little kit on ebay with an adjustable knob so that you can change the readings from the O2 sensor while sitting in your car driving. Does anyone know if this would increase mileage on a cng? Since the cavalier has a separate computer for cng operation, I would expect that this has already been taken into consideration by the cng computer, but who knows...
                  Last edited by KDB4; 09-01-2008, 11:41 AM.

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                  • #10
                    Re: Bad Gas Mileage

                    I have personally driven Cavaliers over 170 miles on CNG. I recently drove a 2001 (with the four speed) from Northern CA to Utah and averaged 30 mpg cruising close to 90mph almost the whole way.

                    If it's all city driving on a decent fill it's usually about 120-130 miles.
                    www.CNGUtah.com

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