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  • Incredible Mileage

    A few weeks ago my son and I went for a trip in central Utah in my 2008 Honda GX. We stopped in at the Richfield CNG station and filled up. I was able to get a fairly good pressure of about 3300 psi. We went West past Loa and the Capitol Reef to a small ghost town called Cainesville. I was more than a little anxious as we got there since we had already put in over a hundred miles on the trip and the drive goes over some pretty high summits of 8-9 thousand feet. I wouldn't be able to add fuel until we returned to Richfield. We headed back and I was surprised to see that the fuel gauge still registered about a quarter tank. I filled up again at the same station and got the same approximate pressure. When I calculated my mileage based on the GGEs I had put into the tank, I was amazed that I had gotten about 57 mpg.

    I've read that mileage can be asymmetrical going uphill and downhill, but I had not considered it to be such a difference. Is there something about the CNG motor or computer characteristics that would cause such a huge boost in mileage when going over mountain ranges? I had been a little careful about pushing the speed going uphill but had really not done anything extreme to save gas.

  • #2
    Re: Incredible Mileage

    Originally posted by dbrewer333 View Post
    A few weeks ago my son and I went for a trip in central Utah in my 2008 Honda GX. We stopped in at the Richfield CNG station and filled up. I was able to get a fairly good pressure of about 3300 psi. We went West past Loa and the Capitol Reef to a small ghost town called Cainesville. I was more than a little anxious as we got there since we had already put in over a hundred miles on the trip and the drive goes over some pretty high summits of 8-9 thousand feet. I wouldn't be able to add fuel until we returned to Richfield. We headed back and I was surprised to see that the fuel gauge still registered about a quarter tank. I filled up again at the same station and got the same approximate pressure. When I calculated my mileage based on the GGEs I had put into the tank, I was amazed that I had gotten about 57 mpg.

    I've read that mileage can be asymmetrical going uphill and downhill, but I had not considered it to be such a difference. Is there something about the CNG motor or computer characteristics that would cause such a huge boost in mileage when going over mountain ranges? I had been a little careful about pushing the speed going uphill but had really not done anything extreme to save gas.
    D,

    If I'm reading your post correctly it sounds like the return trip from Cainsville was more downhill than uphill. Naturally, when you drive downhill your mileage will be better. But, when you fill your tank with it being only 1/4 full to start with, there's a lot of heating of the gas going on. It appears that the tank is full, but when the gas in the tank is allowed to cool, like over night, you can really add more. So, your mileage will drop a bit.

    I've done the same and gotten real good mileage. But when I fill it, drive home, then top it off the next day the mileage drops to a more realistic level. (but still pretty good) When you top off a tank that's 90% full, the gas being added doesn't heat up as much, allowing for a more complete (mo' better?) fill.

    Something that's increased my mileage the greatest amount is to shut the engine off at long red lights and coast downhill when possible.

    Bill
    Last edited by Green GX; 09-02-2008, 09:25 AM. Reason: Just not wordy enough.

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    • #3
      Re: Incredible Mileage

      Was it warmer on your second fillup? If so, you will have gotten less gas at the same pressure and this would calculate out to better mileage, when actually you just got less gas the second time. Just a thought.
      2000 Escalade (option 3 conversion) FOR SALE
      2004 F150 XLT (OEM conversion)
      2000 Camry (Awesome Car!!)

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      • #4
        Re: Incredible Mileage

        Because of the irregularities of CNG fueling, it is impossible to do single tank mileage calculations. It takes adding together all mileage travelled and gas dispensed over about 3-5 fill-ups to average out the irregularities of a single fill-up.
        1997 Factory Crown Victoria w/ extended tanks ~~ Clunkerized!
        2000 Bi-Fuel Expedition --> ~~ Sold ~~ <--

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        • #5
          Re: Incredible Mileage

          If you want to calculate an acurrate MPG, here's what you do:

          Wait until your CNG tank is empty (for a bi-fuel this is easy - when your running on gasoline, for a Civic GX a little more tough - wait until your light has been on for a while). Fill up your tank and keep record of the gallons pumped. Now drive until you run out of CNG (for a bi-fuel this is easy, for a Civic Gx a little more tough). Then you know how far you went on that particular fill up.
          Jared.
          Mountain Green, Utah
          2003 CNG Cavalier
          2003 CNG Silverado 2500HD

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          • #6
            Re: Incredible Mileage

            Thanks for the observations and tips. I measure my MPG at each fillup and of course the variation of driving conditions, gas quality, pressure, etc., all make for quite a difference. Since December, my overall average has been 38 mpg (I commute 60 miles roundtrip each day). I typically see variations of 35-45 mpg, so this variation was certainly an anomaly. What caught my attention was that I got about 210 miles and still had a quarter tank showing before I filled up that afternoon. The temperature variation would make a difference, though I see far less variation normally. I guess what I'm interested in knowing is whether there is indeed an asymmetry with regard to mileage when going uphill or down. You would normally expect that whatever savings you get going downhill would be eaten up going uphill. It doesn't look like that has been the case here. Is that perhaps the reason why hypermilers do the periodic (and illegal) acceleration and coasting?

            Thanks,

            Dave.

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