Why can a vehicle's mileage vary from one full tank to another?

Customers often ask this question, especially when they first start using methane.
Basically, there are two sides to the question, one to do with consumption, the other with the amount of fuel.
Over the same distance, as with all fuels, real consumption depends on both driving style and traffic conditions.
Other factors that affect mileage include engine efficiency and atmospheric conditions (air pressure and humidity).
However, other factors unique to gas come into play when you use methane. Since methane isn't a liquid, the amount you take on board depends not only on the size of your tank (i.e. the capacity in litres of your car's gas cylinders) but also its pressure and temperature when filling is complete.
Though modern facilities ensure ideal filling conditions, the size of a full tank can still vary slightly.
Another very important factor influencing the amount of gas taken on board is the type of gas itself (see the next question).
In any case, the exact amount of gas supplied is indicated at the pump, which is regularly tested and calibrated by the local Weights and Measures Office.
So a certain difference in mileage from the same amount of fuel is normal and evitable, and should be no cause for concern.