Frank van der Hulst wrote:

We get 15 in a van that is always fully loaded with tools all the time.
There is supposedly 10 gallons on board but we go about 120 miles on 7.5
gallons of CNG.

I don't understand what you mean by "10 gallons" of CNG. Does this mean
"10 gallons of petrol equivalent"? Or that the capacity of your gas
cylinders is 10 gallons?

The following is all from memory of about 15 years ago

CNG is a gas, so the volume is meaningless without pressure and
temperature.

CNG approximately follows Boyle's and Charles' laws relating to gas
compression...

PV = nRT

P = Pressure
V = Volume
n = No. moles of gas (quantity, essentially proportional to its weight)
R = constant
T = temperature

This is an approximation -- there is a couple of factors,
super-compressibility and one other whose name I've forgotten, which
means that CNG doesn't exactly conform to the above formula.

On top of that, the composition of CNG varies a little from one day to
the next (at least, it did here in NZ). The most relevant constituents
were methane and CO2. The gas supplier produced a statement each week of
the average gas composition, including the energy content.

I am trying to understand the conversion tables between
compressed CNG and uncompressed as to how it relates to a gallon of regular
fuel.

As you can see from the above, the relationship between a given volume
of gas and its energy content is complex.

If your gas supplier doesn't compensate for temperature when he charges
you, you will get a *lot* more gas for your money if you buy it in the
early morning when the temperature is low.

Frank.

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