fuel mileage calculations

[rla216]

I'm new to the CNG world and this forum.

I have a 2001 Crown Vic I recently bought that does exceptionally well on fuel mileage [26+ hiway]. The factory says it should only get around 19 hiway. I did 2 things that helped. I replaced the air filter box with a K&N box and cone filter. Then I converted the single exhaust to dual. Very simple since the car already has dual cats. Just add the other side with a crossover and additional muffler. This also adds a slight boost in power.

Now to my question. In order to accurately keep a mileage record, I assume I should fill up at the same station with the same pressure. Even then the pressure isn't always consistant. If the pressure differs by much I won't get as much fuel thereby getting an inaccurate measure of mileage. Does anyone know the ratio of GGE's to pressure? In other words, if I fill up at 3400 lbs one time then 3000 lbs the next, what's the differene in volume? [per 100 lbs or whatever]

Thanks

[CNGLO]

Re: fuel mileage calculations

There are many other variables that come into play and that make it almost impossible to get a tank-by-tank reading of your miles per gge. The ambient temperature, the speed of the fill, the temperature of your tank when you begin you fill, etc., etc. It is best to keep a running total of your fills and average over many, many fills.

[mecklen]

Re: fuel mileage calculations

I have a log book in the car and record the mileage and gge for each fill up. Then I average them over the life of the car. It stabilizes after 10 tank fulls. I'm getting ~35 in my '07 GX.

[CraziFuzzy]

Re: fuel mileage calculations

I made up a spreadsheet for it that I enter my odometer reading and amount of fill. It will keep track of my mpgge for that fill-up (erratic) and the average from the last 5 fillups (nice and stable, but lets me see how changes affect the overall efficiency).

[freedml]

Re: fuel mileage calculations

I think CNG dispensers are all over the map with accuracy and even averaging over 5 or more 'tankfuls' tells you more about the accuracy of the dispensers than the mileage you're getting.

[cngmike]

Re: fuel mileage calculations

I think CNG dispensers are all over the map with accuracy and even averaging over 5 or more 'tankfuls' tells you more about the accuracy of the dispensers than the mileage you're getting.

Because of temperature compensation, heat of compression, ambient temperature and fill pressure. the only way to get a good milage average, when fast filling, is over multiple fills.

[CraziFuzzy]

Re: fuel mileage calculations

I think CNG dispensers are all over the map with accuracy and even averaging over 5 or more 'tankfuls' tells you more about the accuracy of the dispensers than the mileage you're getting.

CNG dispensers are regulated by the same requirements that gasoline dispensers are. If you genuinly suspect that it is NOT accurate, you should contact your county's weights and measures department (or equivalent) and file a complaint.

[rla216]

Re: fuel mileage calculations

Thanks, everyone. I guess it's not as sure as gasoline, however I typically fill up early in the morning at the same station with about the same temperature [at least this time of year]. I think with a long term record it will be fairly accurate, as you have all said.

[freedml]

Re: fuel mileage calculations

It will only be as accurate as the GGE display on the pump, which I don't have much faith in.

[cngmike]

Re: fuel mileage calculations

What type of dispenser is it? Dose it have a weights and measurers sticker?

[CraziFuzzy]

Re: fuel mileage calculations

As stated, over multiple fills it WILL be accurate. Over a period of time, you know how many miles you've driver, and how many gallons you've put in. This will give an accurate mileage. The only inaccuracy lies in trying to determine what mileage you got with a particular fill-up, which, in all honesty, doesn't mean squat.